OBD to RS232 Interpreter
Almost all of the automobiles produced today
are required, by law, to provide an interface for the
connection of diagnostic test equipment. The data
transfer on these interfaces follow several standards,
but none of them are directly usable by PCs or smart
devices. The ELM327 is designed to act as a bridge
between these On-Board Diagnostics (OBD) ports
and a standard RS232 interface.
In addition to being able to automatically detect
and interpret nine OBD protocols, the ELM327 also
provides support for high speed communications, a
low power sleep mode, and the J1939 truck and bus
standard. It is also completely customizable, should
you wish to alter it to more closely suit your needs.
The following pages discuss all of the ELM327’s
features in detail, how to use it and configure it, as
well as providing some background information on
the protocols that are supported. There are also
schematic diagrams and tips to help you to interface
to microprocessors, construct a basic scan tool, and
to reduce power consumption.
• Power Control with standby mode
• RS232 baud rates to 500 kbps
• Automatically searches for protocols
• Fully configurable with AT commands
• Low power CMOS design
Connection Diagram
(top view)
J1850 Volts
RS232 Tx LED
J1850 Bus+
RS232 Rx LED
Baud Rate
• Diagnostic trouble code readers
• Automotive scan tools
• Teaching aids
Block Diagram
RS232 Rx
RS232 Tx
PwrCtrl / Busy
J1850 Bus-
IgnMon / RTS
4.00 MHz
Baud Rate
PwrCtrl / Busy
IgnMon / RTS
ISO 15765-4
SAE J1939
ISO 9141-2
ISO 14230-4
SAE J1850
OBD interfaces
status LEDs
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Electrical Information
Pin Descriptions........................................................................... 3
Unused Pins.................................................................................5
Ordering Information.................................................................... 5
Absolute Maximum Ratings......................................................... 5
Electrical Characteristics..............................................................6
Using the ELM327
Overview...................................................................................... 7
Communicating with the ELM327................................................ 7
AT Commands............................................................................. 9
AT Command Summary...............................................................9
AT Command Descriptions........................................................ 11
Reading the Battery Voltage...................................................... 27
OBD Commands........................................................................ 28
Talking to the Vehicle.................................................................29
Interpreting Trouble Codes........................................................ 31
Resetting Trouble Codes........................................................... 32
Quick Guide for Reading Trouble Codes................................... 32
Bus Initiation...............................................................................33
Wakeup Messages.....................................................................33
Selecting Protocols.................................................................... 34
OBD Message Formats..............................................................35
Setting the Headers................................................................... 37
Monitoring the Bus..................................................................... 40
CAN Messages and Filtering..................................................... 41
Multiline Responses................................................................... 42
CAN Message Formats..............................................................44
Restoring Order..........................................................................45
Advanced Features
Altering Flow Control Messages................................................ 46
Using CAN Extended Addresses............................................... 47
SAE J1939 Messages................................................................48
Using J1939............................................................................... 50
The FMS Standard.....................................................................53
Programmable Parameters........................................................ 54
Programmable Parameter Summary......................................... 55
Using Higher RS232 Baud Rates...............................................59
Setting Timeouts - AT ST and AT AT Commands..................... 61
Power Control............................................................................ 62
Design Examples
Microprocessor Interfaces..........................................................64
Example Applications.................................................................65
Modifications for Low Power Standby Operation....................... 70
Error Messages and Alerts.........................................................72
Outline Diagrams....................................................................... 74
Copyright and Disclaimer........................................................... 74
Index.......................................................................................... 75
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Pin Descriptions
MCLR (pin 1)
A momentary (>2µsec) logic low applied to this input
will reset the ELM327. If unused, this pin should be
connected to a logic high (VDD) level.
Vmeasure (pin 2)
This analog input is used to measure a 0 to 5V
signal that is applied to it. Care must be taken to
prevent the voltage from going outside of the supply
levels of the ELM327, or damage may occur. If it is
not used, this pin should be tied to either VDD or VSS.
J1850 Volts (pin 3)
This output can be used to control a voltage supply
for the J1850 Bus+ output. The pin normally outputs
a logic high level when a nominal 8V is required (for
J1850 VPW), and a low level for 5V (for J1850
PWM), but this can be changed with PP 12. If this
switching capability is not required for your
application, this output can be left open-circuited.
J1850 Bus+ (pin 4)
This active high output is used to drive the
J1850 Bus+ Line to an active level. Note that this
signal does not have to be used for the Bus- Line (as
was the case for the ELM320), since a separate
J1850 Bus- drive output is provided on pin 14.
return and a linefeed character. If it is at a low level,
lines will be terminated by a carriage return only.
This behaviour can always be modified by issuing an
AT L1 or AT L0 command.
VSS (pin 8)
Circuit common must be connected to this pin.
XT1 (pin 9) and XT2 (pin 10)
A 4.000 MHz oscillator crystal is connected between
these two pins. Loading capacitors as required by
the crystal (typically 27pF each) will also need to be
connected between each of these pins and circuit
common (Vss).
Note that this device has not been configured for
operation with an external oscillator – it expects a
crystal to be connected to these pins. Use of an
external clock source is not recommended. Also,
note that this oscillator is turned off when in the Low
Power or ‘standby’ mode of operation.
VPW In (pin 11)
This is the active high input for the J1850 VPW data
signal. When at rest (bus recessive) this pin should
be at a low logic level. This input has Schmitt trigger
wave shaping, so no special amplification is
Memory (pin 5)
This input controls the default state of the memory
option. If this pin is at a high level during power-up or
reset, the memory function will be enabled by
default. If it is at a low level, then the default will be
to have it disabled. Memory can always be enabled
or disabled with the AT M1 and AT M0 commands.
ISO In (pin 12)
This is the active low input for the ISO 9141 and
ISO 14230 data signal. It is derived from the K Line,
and should be at a high logic level when at rest (bus
recessive). No special amplification is required, as
this input has Schmitt trigger wave shaping.
Baud Rate (pin 6)
This input controls the baud rate of the RS232
interface. If it is at a high level during power-up or
reset, the baud rate will be set to 38400 (or the rate
that has been set by PP 0C). If at a low level, the
baud rate will always be 9600.
PWM In (pin 13)
This is the active low input for the J1850 PWM data
signal. It should normally be at a high level when at
rest (ie. bus recessive). This input has Schmitt
trigger wave shaping, so no special amplification is
LFmode (pin 7)
This input is used to select the default linefeed mode
to be used after a power-up or system reset. If it is at
a high level, then by default messages sent by the
ELM327 will be terminated with both a carriage
J1850 Bus- (pin 14)
This active high output is used to drive the J1850
Bus- Line to an active (dominant) level for J1850
PWM applications. If unused, this output can be left
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Pin Descriptions (continued)
IgnMon / RTS (pin 15)
This input pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
If both bit 7 and bit 2 of PP 0E are ‘1’s, this pin will
act as an Ignition Monitor. This will result in a switch
to the Low Power mode of operation, should the
signal go to a low level, as would happen if the
vehicle’s ignition were turned off. An internal
‘debounce’ timer is used to ensure that the ELM327
does not shut down for noise at the input.
When the voltage at pin 15 is again restored to a
high level, and a time of 1 or 5 seconds (as set by
PP 0E bit 1) passes, the ELM327 will perform a
‘Warm Start’ and return to normal operation. A low to
high transition at pin 15 will in fact restore normal
operation, regardless of the setting of PP 0E bit 2, or
whether pin 15 was the initial cause for the low
power mode. This feature allows a system to control
how and when it switches to low power standby
operation, but still have automatic wakeup by the
ignition voltage, or even by a pushbutton.
If either bit 7 or bit 2 of PP 0E are ‘0’, this pin will
function as an active low ‘Request To Send’ input.
This can be used to interrupt the OBD processing in
order to send a new command, or as previously
mentioned, to highlight the fact that the ignition has
been turned off. Normally kept at a high level, this
input is brought low for attention, and should remain
so until the Busy line (pin 16) indicates that the
ELM327 is no longer busy, or until a prompt
character is received (if pin 16 is being used for
power control).
This input has Schmitt trigger wave shaping. By
default, pin 15 acts as the RTS interrupt input.
PwrCtrl / Busy (pin 16)
This output pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
If bit 7 of PP 0E is a ‘1’ (the default), this pin will
function as a Power Control output. The normal state
of the pin will be as set by PP 0E bit 6, and the pin
will remain in that state until the ELM327 switches to
the Low Power mode of operation, when the output
changes to the opposite level. This output is typically
used to control enable inputs, but may also be used
for relay circuits, etc. with suitable buffering. The
discussion on page 70 (‘Modifications for Low Power
Standby Operation’) provides more detail on how to
use this output.
If bit 7 of PP 0E is a ‘0’, pin 16 will function as a
‘Busy’ output, showing when the ELM327 is actively
processing a command (the output will be at a high
level), or when it is idle, ready to receive commands
(the output will be low).
By default, bit 7 of PP 0E is ‘1’, so pin 16 provides
the Power Control function.
RS232Tx (pin 17)
This is the RS232 data transmit output. The signal
level is compatible with most interface ICs (the
output is high when idle), and there is sufficient
current drive to allow interfacing using only a PNP
transistor, if desired.
RS232Rx (pin 18)
This is the RS232 receive data input. The signal
level is compatible with most interface ICs (when at
idle, the level should be high), but can be used with
other interfaces as well, since the input has Schmitt
trigger wave shaping.
VSS (pin 19)
Circuit common must be connected to this pin.
VDD (pin 20)
This pin is the positive supply pin, and should always
be the most positive point in the circuit. Internal
circuitry connected to this pin is used to provide
power on reset of the microprocessor, so an external
reset signal is not required. Refer to the Electrical
Characteristics section for further information.
ISO K (pin 21) and ISO L (pin 22)
These are the active high output signals which are
used to drive the ISO 9141 and ISO 14230 buses to
an active (dominant) level. Many new vehicles do not
require the L Line – if yours does not, you can simply
leave pin 22 open-circuited.
CAN Tx (pin 23) and CAN Rx (pin 24)
These are the two CAN interface signals that must
be connected to a CAN transceiver IC (see the
Example Applications section for more information).
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Pin Descriptions (continued)
If unused, pin 24 must be connected to a logic high
(VDD) level.
RS232 Rx LED (pin 25), RS232 Tx LED (pin 26),
OBD Rx LED (pin 27) and OBD Tx LED (pin 28)
These four output pins are normally high, and are
driven to low levels when the ELM327 is transmitting
or receiving data. These outputs are suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic circuits. If
unused, these pins may be left open-circuited.
Note that pin 28 can also be used to turn off all of the
Programmable Parameters, if you can not do so by
using the normal interface - see pages 54 and 55 for
more details.
Unused Pins
When people only want to implement a portion of what the ELM327 is capable of, they often ask what to do with the
unused pins. The rule is that unused outputs may be left open-circuited with nothing connected to them, but unused
inputs must be terminated. The ELM327 is a CMOS integrated circuit that can not have any inputs left floating (or
you might damage the IC). Connect unused inputs as follows:
Note that the inputs that are shown with an asterisk (*) may be connected to either a High (VDD) or a Low (VSS)
level, but the level shown is preferred.
Ordering Information
These integrated circuits are 28 pin devices, available in either a 300 mil wide plastic (‘skinny’) DIP format or in a
300 mil (7.50 mm body) SOIC surface mount type of package. We do not offer an option for QFN packages.
To order, add the appropriate suffix to the part number:
300 mil 28 pin Plastic DIP..............................ELM327P
300 mil 28 pin SOIC....................................ELM327SM
Absolute Maximum Ratings
Storage Temperature....................... -65°C to +150°C
Ambient Temperature with
Power Applied....................................-40°C to +85°C
Voltage on VDD with respect to VSS..... -0.3V to +7.5V
These values are given as a design guideline only.
The ability to operate to these levels is neither
inferred nor recommended, and stresses beyond
those listed here will likely damage the device.
Voltage on any other pin with
respect to VSS........................... -0.3V to (VDD + 0.3V)
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Electrical Characteristics
All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Supply voltage, VDD
VDD rate of rise
Average current, IDD
low power
Input logic levels
Schmitt trigger
input thresholds
Maximum Units
see note 2
ELM327 device only - does not
include any load currents
Pins 5, 6, 7, and 24 only
Pins 1, 11, 12, 13, 15 and 18 only
Output low voltage
current (sink) = 10 mA
Output high voltage
current (source) = 10 mA
Brown-out reset voltage
A/D conversion time
Pin 18 low level pulse duration to
wake the IC from Low Power mode
IgnMon debounce time
see note 3
AT LP to PwrCtrl output time
LP ALERT to PwrCtrl output time
1. This integrated circuit is based on Microchip Technology Inc.’s PIC18F2480 device. For more detailed
device specifications, and possibly clarification of those given, please refer to the Microchip documentation
(available at http://www.microchip.com/).
2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved
using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as
may be obtained through direct connection to solar cells or some charge pump circuits.
3. This is the time between when the AT RV command is received, and when the voltage reading response
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The following describes how to use the ELM327 to
obtain information from your vehicle.
We begin by discussing just how to ‘talk’ to the IC
using a PC, then explain how to change options using
‘AT’ commands, and finally we show how to use the
ELM327 to obtain trouble codes (and reset them). For
the more advanced experimenters, there are also
sections on how to use some of the programmable
features of this product as well.
Using the ELM327 is not as daunting as it first
seems. Many users will never need to issue an ‘AT’
command, adjust timeouts, or change the headers. For
most, all that is required is a PC or smart device with a
terminal program (such as HyperTerminal or ZTerm),
and a little knowledge of OBD commands, which we
will provide in the following sections…
Communicating with the ELM327
The ELM327 expects to communicate with a PC
through an RS232 serial connection. Although modern
computers do not usually provide a serial connection
such as this, there are several ways in which a ‘virtual
serial port’ can be created. The most common devices
are USB to RS232 adapters, but there are several
others such as PC cards, ethernet devices, or
Bluetooth to serial adapters.
No matter how you physically connect to the
ELM327, you will need a way to send and receive
data. The simplest method is to use one of the many
‘terminal’ programs that are available (HyperTerminal,
ZTerm, etc.), to allow typing the characters directly
from your keyboard.
To use a terminal program, you will need to adjust
several settings. First, ensure that your software is set
to use the proper ‘COM’ port, and that you have
chosen the proper data rate - this will be either 9600
baud (if pin 6 = 0V at power up), or 38400 baud (if
PP 0C has not been changed). If you select the wrong
‘COM’ port, you will not be able to send or receive any
data. If you select the wrong data rate, the information
that you send and receive will be all garbled, and
unreadable by you or the ELM327. Don’t forget to also
set your connection for 8 data bits, no parity bits, and 1
stop bit, and to set it for the proper ‘line end’ mode. All
of the responses from the ELM327 are terminated with
a single carriage return character and, optionally, a
linefeed character (depending on your settings).
Properly connected and powered, the ELM327 will
energize the four LED outputs in sequence (as a lamp
test) and will then send the message:
ELM327 v1.4b
In addition to identifying the version of this IC,
receiving this string is a good way to confirm that the
computer connections and terminal software settings
are correct (however, at this point no communications
have taken place with the vehicle, so the state of that
connection is still unknown).
The ‘>’ character that is shown on the second line
is the ELM327’s prompt character. It indicates that the
device is in the idle state, ready to receive characters
on the RS232 port. If you did not see the identification
string, you might try resetting the IC again with the AT
Z (reset) command. Simply type the letters A T and Z
(spaces are optional), then press the return key:
That should cause the leds to flash again, and the
identification string to be printed. If you see strange
looking characters, then check your baud rate - you
have likely set it incorrectly.
Characters sent from the computer can either be
intended for the ELM327’s internal use, or for
reformatting and passing on to the vehicle. The
ELM327 can quickly determine where the received
characters are to be directed by monitoring the
contents of the message. Commands that are
intended for the ELM327’s internal use will begin with
the characters ‘AT’, while OBD commands for the
vehicle are only allowed to contain the ASCII codes for
hexadecimal digits (0 to 9 and A to F).
Whether it is an ‘AT’ type internal command or a
hex string for the OBD bus, all messages to the
ELM327 must be terminated with a carriage return
character (hex ‘0D’) before it will be acted upon. The
one exception is when an incomplete string is sent and
no carriage return appears. In this case, an internal
timer will automatically abort the incomplete message
after about 20 seconds, and the ELM327 will print a
single question mark (‘?’) to show that the input was
not understood (and was not acted upon).
Messages that are not understood by the ELM327
(syntax errors) will always be signalled by a single
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Communicating with the ELM327 (continued)
question mark. These include incomplete messages,
incorrect AT commands, or invalid hexadecimal digit
strings, but are not an indication of whether or not the
message was understood by the vehicle. One must
keep in mind that the ELM327 is a protocol interpreter
that makes no attempt to assess the OBD messages
for validity – it only ensures that hexadecimal digits
were received, combined into bytes, then sent out the
OBD port, and it does not know if a message sent to
the vehicle was in error.
While processing OBD commands, the ELM327
will continually monitor for either an active RTS input,
or an RS232 character received. Either one will
interrupt the IC, quickly returning control to the user,
while possibly aborting any initiation, etc. that was in
progress. After generating a signal to interrupt the
ELM327, software should always wait for either the
prompt character (‘>’ or hex 3E), or a low level on the
Busy output before beginning to send the next
Finally, it should be noted that the ELM327 is not
case-sensitive, so the commands ‘ATZ’, ‘atz’, and
‘AtZ’ are all exactly the same to the ELM327. All
commands may be entered as you prefer, as no one
method is faster or better. The ELM327 also ignores
space characters and all control characters (tab, etc.),
so they can be inserted anywhere in the input if that
improves readability.
One other feature of the ELM327 is the ability to
repeat any command (AT or OBD) when only a single
carriage return character is received. If you have sent
a command (for example, 01 0C to obtain the rpm),
you do not have to resend the entire command in
order to resend it to the vehicle - simply send a
carriage return character, and the ELM327 will repeat
the command for you. The memory buffer only
remembers the one command - there is no provision in
the current ELM327 to provide storage for any more.
Please Note:
There is a very small chance that NULL characters (byte value 00) may occasionally
be inserted into the RS232 data that is transmitted by the ELM327.
Microchip Technology has reported that some ICs which use the same EUSART as
in the ELM327 may, under very specific (and rare) conditions, insert an extra byte
(always of value 00) into the transmitted data. If you are using a terminal program to view
the data, you should select the ‘hide control characters’ option if it is available, and if you
are writing software for the ELM327, then monitor incoming bytes, and ignore any that
are of value 00 (ie. remove NULLs).
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AT Commands
Several parameters within the ELM327 can be
adjusted in order to modify its behaviour. These do not
normally have to be changed before attempting to talk
to the vehicle, but occasionally the user may wish to
customize these settings – for example by turning the
character echo off, adjusting a timeout value, or
changing the header bytes. In order to do this, internal
‘AT’ commands must be used.
Those familiar with PC modems will immediately
recognize AT commands as a standard way in which
modems are internally configured. The ELM327 uses
essentially the same method, always watching the
data sent by the PC, looking for messages that begin
with the character ‘A’ followed by the character ‘T’. If
found, the next characters will be interpreted as an
internal configuration or ‘AT’ command, and will be
executed upon receipt of a terminating carriage return
character. If the command is just a setting change, the
ELM327 will reply with the characters ‘OK’, to say that
it was successfully completed.
Some of the following commands allow passing
numbers as arguments in order to set the internal
values. These will always be hexadecimal numbers
which must generally be provided in pairs. The
hexadecimal conversion chart in the OBD Commands
section (page 28) may be helpful if you wish to
interpret the values. Also, one should be aware that for
the on/off types of commands, the second character is
the number 1 or the number 0, the universal terms for
on and off.
The remainder of this page, and the next page
following provide a summary of all of the commands
that the current version of the ELM327 recognizes. A
more complete description of each command begins
on page 11.
AT Command Summary
General Commands
Programmable Parameter Commands
repeat the last command
BRD hh
try Baud Rate Divisor hh
BRT hh
set Baud Rate Timeout
set all to Defaults
E0, E1
Echo off, or on*
Forget Events
print the version ID
L0, L1
Linefeeds off, or on
go to Low Power mode
M0, M1
Memory off, or on
Read the stored Data
SD hh
Save Data byte hh
Warm Start (quick software reset)
reset all
display the device description
display the device identifier
@3 cccccccccccc store the @2 identifier
PP xx OFF disable Prog Parameter xx
PP FF OFF all Prog Parameters disabled
PP xx ON enable Prog Parameter xx
PP FF ON all Prog Parameters enabled
PP xx SV yy for PP xx, Set the Value to yy
print a PP Summary
Voltage Reading Commands
CV dddd
CV 0000
Calibrate the Voltage to dd.dd volts
restore CV value to factory setting
Read the input Voltage
read the IgnMon input level
Settings shown with an asterisk (*)
are the default values
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AT Command Summary (continued)
OBD Commands
AT0, 1, 2
H0, H1
MR hh
MT hh
R0, R1
RA hh
S0, S1
SH xyz
SH xxyyzz
SP h
SR hh
ST hh
TA hh
TP h
J1850 Specific Commands (protocols 1 and 2)
Allow Long (>7 byte) messages
Automatically Receive
Adaptive Timing off, auto1*, auto2
perform a Buffer Dump
Bypass the Initialization sequence
Describe the current Protocol
Describe the Protocol by Number
Headers off*, or on
Monitor All
Monitor for Receiver = hh
Monitor for Transmitter = hh
Normal Length messages*
Protocol Close
Responses off, or on*
set the Receive Address to hh
printing of Spaces off, or on*
Set Header to xyz
Set Header to xxyyzz
Set Protocol to h and save it
Set Protocol to Auto, h and save it
Set the Receive address to hh
use Standard Search order (J1978)
Set Timeout to hh x 4 msec
set Tester Address to hh
Try Protocol h
Try Protocol h with Auto search
ISO Specific Commands (protocols 3 to 5)
perform a Fast Initiation
IB 10
set the ISO Baud rate to 10400*
IB 48
set the ISO Baud rate to 4800
IB 96
set the ISO Baud rate to 9600
IIA hh
set ISO (slow) Init Address to hh
display the Key Words
KW0, KW1 Key Word checking off, or on*
perform a Slow (5 baud) Initiation
SW hh
Set Wakeup interval to hh x 20 msec
WM [1 - 6 bytes] set the Wakeup Message
IFR0, 1, 2
IFRs off, auto*, or on
IFR value from Header* or Source
CAN Specific Commands (protocols 6 to C)
turn off CAN Extended Addressing
CEA hh
use CAN Extended Address hh
CAF0, CAF1 Automatic Formatting off, or on*
CF hhh
set the ID Filter to hhh
CF hhhhhhhh set the ID Filter to hhhhhhhh
CFC0, CFC1 Flow Controls off, or on*
CM hhh
set the ID Mask to hhh
CM hhhhhhhh set the ID Mask to hhhhhhhh
CP hh
set CAN Priority to hh (29 bit)
reset the Receive Address filters
CRA hhh
set CAN Receive Address to hhh
CRA hhhhhhhh set the Rx Address to hhhhhhhh
show the CAN Status counts
CSM0, CSM1 Silent Monitoring off, or on*
D0, D1
display of the DLC off*, or on
Flow Control, Set the Mode to h
FC SH hhh
FC, Set the Header to hhh
FC SH hhhhhhhh Set the Header to hhhhhhhh
FC SD [1 - 5 bytes] FC, Set Data to [...]
PB xx yy
Protocol B options and baud rate
send an RTR message
V0, V1
use of Variable DLC off*, or on
J1939 CAN Specific Commands (protocols A to C)
MP hhhh
MP hhhh n
MP hhhhhh
MP hhhhhh n
Elm Electronics – Circuits for the Hobbyist
monitor for DM1 messages
use J1939 Elm data format*
Header Formatting off, or on*
use J1939 SAE data format
set Timer Multiplier to 1*
set Timer Multiplier to 5
Monitor for PGN 0hhhh
“ “ and get n messages
Monitor for PGN hhhhhh
“ “ and get n messages
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AT Command Descriptions
The following describes each AT Command that the
current version of the ELM327 supports:
[ Allow Long messages ]
The standard OBDII protocols restrict the number
of data bytes in a message to seven, which the
ELM327 normally does as well (for both send and
receive). If AL is selected, the ELM327 will allow long
sends (eight data bytes) and long receives (unlimited
in number). The default is AL off (and NL selected).
is always as set by AT ST), while AT2 is a more
aggressive version of AT1 (the effect is more
noticeable for very slow connections – you may not
see much difference with faster OBD systems). The
J1939 protocol does not support Adaptive Timing – it
uses fixed timeouts as set in the standard.
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM327, if the internally stored
receive address matches the address that the
message is being sent to. With the auto receive mode
in effect, the value used for the receive address will be
chosen based on the current header bytes, and will
automatically be updated whenever the header bytes
are changed.
The value that is used for the receive address is
determined based on such things as the contents of
the first header byte, and whether the message uses
physical addressing, functional addressing, or if the
user has set a value with the SR or RA commands.
Auto Receive is turned on by default, and is not
used by the J1939 protocol.
AT0, AT1 and AT2
[ Adaptive Timing control ]
When receiving responses from a vehicle, the
ELM327 has traditionally waited the time set by the
AT ST hh setting for a response. To ensure that the IC
would work with a wide variety of vehicles, the default
value was set to a conservative (slow) value. Although
it was adjustable, many people did not have the
equipment or experience to determine a better value.
The Adaptive Timing feature automatically sets the
timeout value for you, to a value that is based on the
actual response times that your vehicle is responding
in. As conditions such as bus loading, etc. change, the
algorithm learns from them, and makes appropriate
adjustments. Note that it always uses your AT ST hh
setting as the maximum setting, and will never choose
one which is longer.
There are three adaptive timing settings that are
available for use. By default, Adaptive Timing option 1
(AT1) is enabled, and is the recommended setting.
AT0 is used to disable Adaptive timing (so the timeout
[ perform an OBD Buffer Dump ]
All messages sent and received by the ELM327
are stored temporarily in a set of twelve memory
storage locations called the OBD Buffer. Occasionally,
it may be of use to view the contents of this buffer,
perhaps to see why an initiation failed, to see the
header bytes in the last message, or just to learn more
of the structure of OBD messages. You can ask at any
time for the contents of this buffer to be ‘dumped’
(ie printed) – when you do, the ELM327 sends a length
byte (representing the length of the message in the
buffer) followed by the contents of all twelve OBD
buffer locations. For example, here’s one ‘dump’:
05 C1 33 F1 3E 23 C4 00 00 10 F8 00 00
The 05 is the length byte - it tells us that only the
following 5 bytes (C1 33 F1 3E and 23) are valid. The
remaining bytes are likely left over from a previous
The length byte always represents the actual
number of bytes received, whether they fit into the
OBD buffer or not. This may be useful when viewing
long data streams (with AT AL), as it represents the
actual number of bytes received, mod 256. Note that
only the first twelve bytes received are stored in the
[ Bypass the Initialization sequence ]
This command should be used with caution. It
allows an OBD protocol to be made active without
requiring any sort of initiation or handshaking to occur.
The initiation process is normally used to validate the
protocol, and without it, results may be difficult to
predict. It should not be used for routine OBD use, and
has only been provided to allow the construction of
ECU simulators and training demonstrators.
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AT Command Descriptions (continued)
BRD hh
[ try Baud Rate Divisor hh ]
This command is used to change the RS232 baud
rate divisor to the hex value provided by hh, while
under computer control. It is not intended for casual
experimenting - if you wish to change the baud rate
from a terminal program, you should use PP 0C.
Since some interface circuits are not able to
operate at high data rates, the BRD command uses a
sequence of sends and receives to test the interface,
with any failure resulting in a fallback to the previous
baud rate. This allows several baud rates to be tested
and a reliable one chosen for the communications.
The entire process is described in detail in the ‘Using
Higher RS232 Baud Rates’ section, on pages 59 and
If successful, the actual baud rate (in kbps) will be
4000 divided by the divisor (hh).
BRT hh
[ set Baud Rate Timeout to hh ]
This command allows the timeout used for the
Baud Rate handshake (ie. AT BRD) to be varied. The
time delay is given by hh x 5.0 msec, where hh is a
hexadecimal value. The default value for this setting is
0F, providing 75 msec. Note that a value of 00 does
not result in 0 msec - it provides the maximum time of
256 x 5.0 msec, or 1.28 seconds.
CAF0 and CAF1
[ CAN Auto Formatting off or on ]
These commands determine whether the ELM327
assists you with the formatting of the CAN data that is
sent and received. With CAN Automatic Formatting
enabled (CAF1), the IC will automatically generate the
formatting (PCI) bytes for you when sending, and will
remove them when receiving. This means that you can
continue to issue OBD requests (01 00, etc.) as usual,
without regard to the extra bytes that CAN diagnostics
systems require. Also, with formatting on, any extra
(unused) data bytes that are received in the frame will
be removed, and any messages with invalid PCI bytes
will be ignored. (When monitoring, however, messages
with invalid PCI bytes are shown, with a ‘<DATA
ERROR’ message beside them).
Multi-frame responses may be returned by the
vehicle with ISO 15765 and J1939. To make these
more readable, the Auto Formatting mode will extract
the total data length and print it on one line, then show
each line of data with the segment number followed by
a colon (‘:’), and then the data bytes.
You may also see the characters 'FC:' on a line (if
you are experimenting). This identifies a Flow Control
message that has been sent as part of the multi-line
message signalling. Flow Control messages are
automatically generated by the ELM327 in response to
a ‘First Frame’ reply, as long as the CFC setting is on
(it does not matter if auto formatting is on or not).
Another type of message – the RTR (or ‘Remote
Transfer Request’) – will be automatically hidden for
you when in the CAF1 mode, since they contain no
data. When auto formatting is off (CAF0), you will see
the characters 'RTR' printed when a remote transfer
request frame has been received.
Turning the CAN Automatic Formatting off (CAF0),
will cause the ELM327 to print all of the received data
bytes. No bytes will be hidden from you, and none will
be inserted for you. Similarly, when sending a data
request with formatting off, you must provide all of the
required data bytes exactly as they are to be sent –
the ELM327 will not perform any formatting for you
other than to add some trailing 'padding' bytes to
ensure that the required eight data bytes are sent. This
allows operation in systems that do not use PCI bytes
as ISO 15765-4 does.
Note that turning the display of headers on (with
AT H1) will override some of the CAF1 formatting of
the received data frames, so that the received bytes
will appear much like in the CAF0 mode (ie. as
received). It is only the printing of the received data
that will be affected when both CAF1 and H1 modes
are enabled, though; when sending data, the PCI byte
will still be created for you and padding bytes will still
be added. Auto Formatting on (CAF1) is the default
setting for the ELM327.
[ turn off the CAN Extended Address ]
The CEA command is used to turn off the special
features that are set with the CEA hh command.
CEA hh
[ set the CAN Extended Address to hh ]
Some CAN protocols extend the addressing fields
by using the first of the eight data bytes as a target or
receiver’s address. This type of formatting does not
comply with any OBD standard, but by adding it, we
allow for some experimentation.
Sending the CEA hh command causes the
ELM327 to insert the hh value as the first data byte of
all CAN messages that you send. It also adds one
more filtering step to received messages, only passing
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AT Command Descriptions (continued)
ones that have the Tester Address in the first byte
position (in addition to requiring that ID bits match the
patterns set by AT CF and CM, or CRA). The AT CEA
hh command can be sent at any time, and changes
are effective immediately, allowing for changes of the
address ‘on-the-fly’. There is a more lengthy
discussion of extended addressing in the ‘Using CAN
Extended Addresses’ section on page 47.
The CEA mode of operation is off by default, and
once on, can be turned off at any time by sending AT
CEA, with no address. Note that the CEA setting has
no effect when J1939 formatting is on.
CF hhh
[ set the CAN ID Filter to hhh ]
The CAN Filter works in conjunction with the CAN
Mask to determine what information is to be accepted
by the receiver. As each message is received, the
incoming CAN ID bits are compared to the CAN Filter
bits (when the mask bit is a ‘1’). If all of the relevant
bits match, the message will be accepted, and
processed by the ELM327, otherwise it will be
discarded. This three nibble version of the CAN Filter
command makes it a little easier to set filters with 11
bit ID CAN systems. Only the rightmost 11 bits of the
provided nibbles are used, and the most significant bit
is ignored. The data is actually stored as four bytes
internally however, with this command adding leading
zeros for the other bytes. See the CM command(s) for
more details.
CF hh hh hh hh [ set the CAN ID Filter to hhhhhhhh ]
This command allows all four bytes (actually 29
bits) of the CAN Filter to be set at once. The 3 most
significant bits will always be ignored, and may be
given any value. This command may be used to enter
11 bit ID filters as well, since they are stored in the
same locations internally (entering AT CF 00 00 0h hh
is exactly the same as entering the shorter AT CF hhh
CFC0 and CFC1
[ CAN Flow Control off or on ]
The ISO 15765-4 CAN protocol expects a ‘Flow
Control’ message to always be sent in response to a
‘First Frame’ message, and the ELM327 automatically
sends these without any intervention by the user. If
experimenting with a non-OBD system, it may be
desirable to turn this automatic response off, and the
AT CFC0 command has been provided for that
purpose. The default setting is CFC1 - Flow Controls
Note that during monitoring (AT MA, MR, or MT),
there are never any Flow Controls sent no matter what
the CFC option is set to.
CM hhh
[ set the CAN ID Mask to hhh ]
There can be a great many messages being
transmitted in a CAN system at any one time. In order
to limit what the ELM327 views, there needs to be a
system of filtering out the relevant ones from all the
others. This is accomplished by the filter, which works
in conjunction with the mask. A mask is a group of bits
that show the ELM327 which bits in the filter are
relevant, and which ones can be ignored. A ‘must
match’ condition is signalled by setting a mask bit to
'1', while a 'don't care' is signalled by setting a bit to '0'.
This three digit variation of the CM command is used
to provide mask values for 11 bit ID systems (the most
significant bit is always ignored).
Note that a common storage location is used
internally for the 29 bit and 11 bit masks, so an 11 bit
mask could conceivably be assigned with the next
command (CM hh hh hh hh), should you wish to do the
extra typing. The values are right justified, so you
would need to provide five leading zeros followed by
the three mask bytes.
CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]
This command is used to assign mask values for
29 bit ID systems. See the discussion under the
CM hhh command as it is essentially identical, except
for the length. Note that the three most significant bits
that you provide in the first digit will be ignored.
CP hh
[ set CAN Priority bits to hh ]
This command is used to assign the five most
significant bits of the 29 bit CAN ID that is used for
sending messages (the other 24 bits are set with the
AT SH command). Many systems use these bits to
assign a priority value to messages, and to determine
the protocol. Any bits provided in excess of the five
required are ignored, and not stored by the ELM327 (it
only uses the five least significant bits of this byte).
The default value for these priority bits is hex 18,
which can be restored at any time with the AT D
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AT Command Descriptions (continued)
[reset the CAN Rx Addr]
The AT CRA command is used to restore the CAN
receive filters to their default values. Note that it does
not have any arguments (ie no data).
CRA hhh
[set the CAN Rx Addr to hhh]
Setting the CAN masks and filters can be difficult
at times, so if you only want to receive information
from one address (ie. one CAN ID), then this
command may be very welcome. For example, if you
only want to see information from 7E8, simply send AT
CRA 7E8, and the ELM327 will make the necessary
adjustments to both the mask and the filter for you.
Note that this command restricts viewing to only
the one ID - to allow reception of a range of IDs, you
must set the mask and filter independently (described
in detail on page 41). To reverse the changes made by
the CRA command, simply send AT CRA or AT AR.
CRA hhhhhhhh [set the CAN Rx Addr to hhhhhhhh]
This command is identical to the previous one,
except that it is used to set 29 bit CAN IDs, instead of
11. Sending either AT CRA or AT AR will also reverse
the changes made by this command.
[ show the CAN Status counts ]
The CAN protocol requires that statistics be kept
regarding the number of transmit and receive errors
detected. If there should be a significant number of
errors (due to a hardware or software problem), the
device will go off-line in order to not affect other data
on the bus. The AT CS command lets you see both
the transmitter (Tx) and the receiver (Rx) error counts,
in hexadecimal. If the transmitter should be off (count
>FF), you will see ‘OFF’ rather than a specific count.
CSM0 and CSM1
[ CAN Silent Monitoring off or on ]
The ELM327 was designed to be completely silent
while monitoring a CAN bus. Because of this, it is able
to report exactly what it sees, without colouring the
information in any way. Occasionally (when bench
testing, or when connecting to a dedicated CAN port),
it may be preferred that the ELM327 does not operate
silently (ie generates ACK bits, etc.), and this is what
the CSM command is for. CSM1 turns it on, CSM0
turns it off, and the default value is determined by
PP 21. Be careful when experimenting with this. If you
should choose the wrong baud rate then monitor the
CAN bus with the silent monitoring off, you will disturb
the flow of data. Always keep the silent monitoring on
until you are certain that you have chosen the correct
baud rate.
CV dddd
[ Calibrate the Voltage to dd.dd volts ]
The voltage reading that the ELM327 shows for an
AT RV request can be calibrated with this command.
The argument (‘dddd’) must always be provided as 4
digits, with no decimal point (it assumes that the
decimal place is between the second and the third
To use this feature, simply use an accurate meter
to read the actual input voltage, then use the CV
command to change the internal calibration (scaling)
factor. For example, if the ELM327 shows the voltage
as 12.2V while you measure 11.99 volts, then send
AT CV 1199 and the ELM327 will recalibrate itself for
that voltage (it will actually read 12.0V due to digit
roundoff). See page 27 for some more information on
how to read voltages and perform the calibration.
CV 0000
[ restore the factory Calibration Value ]
If you are experimenting with the CV dddd
command but do not have an accurate voltmeter as a
reference, you may soon get into trouble. If this
happens, you can always send AT CV 0000 to restore
the ELM327 to the original calibration value.
[ set all to Defaults ]
This command is used to set the options to their
default (or factory) settings, as when power is first
applied. The last stored protocol will be retrieved from
memory, and will become the current setting (possibly
closing other protocols that are active). Any settings
that the user had made for custom headers, filters, or
masks will be restored to their default values, and all
timer settings will also be restored to their defaults.
D0 and D1
[ display of DLC off or on ]
Standard CAN (ISO 15765-4) OBD requires that
all messages have 8 data bytes, so displaying the
number of data bytes (the DLC) is not normally very
useful. When experimenting with other protocols,
however, it may be useful to be able to see what the
data lengths are. The D0 and D1 commands control
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AT Command Descriptions (continued)
the display of the DLC digit (the headers must also be
on in order to see this digit). When displayed, the
single DLC digit will appear between the ID (header)
bytes and the data bytes. The default setting is
determined by PP 29.
[ monitor for DM1s ]
The SAE J1939 Protocol broadcasts trouble codes
periodically, by way of Diagnostic Mode 1 (DM1)
messages. This command sets the ELM327 to
continually monitor for this type of message for you,
following multi-segment transport protocols as
required. Note that a combination of masks and filters
could be set to provide a similar output, but they would
not allow multiline messages to be detected. The DM1
command adds the extra logic that is needed for
multiline messages.
This command is only available when a CAN
Protocol (A, B, or C) has been selected for J1939
formatting. It returns an error if attempted under any
other conditions.
[ Describe the current Protocol ]
The ELM327 automatically detects a vehicle’s
OBD protocol, but does not normally report what it is.
The DP command is a convenient means of asking
what protocol the IC is currently set to (even if it has
not yet ‘connected’ to the vehicle).
If a protocol is chosen and the automatic option is
also selected, AT DP will show the word 'AUTO' before
the protocol description. Note that the description
shows the actual protocol names, not the numbers
used by the protocol setting commands.
[ Describe the Protocol by Number ]
This command is similar to the DP command, but
it returns a number which represents the current
protocol. If the automatic search function is also
enabled, the number will be preceded with the letter
‘A’. The number is the same one that is used with the
set protocol and test protocol commands.
E0 and E1
[ Echo off or on ]
These commands control whether or not the
characters received on the RS232 port are echoed
(retransmitted) back to the host computer. Character
echo can be used to confirm that the characters sent
to the ELM327 were received correctly. The default is
E1 (or echo on).
FC SD [1-5 bytes]
[ Flow Control Set Data to… ]
The data bytes that are sent in a CAN Flow
Control message may be defined with this command.
One to five data bytes may be specified, with the
remainder of the data bytes in the message being
automatically set to the default CAN filler byte, if
required by the protocol. Data provided with this
command is only used when Flow Control modes 1 or
2 have been enabled.
FC SH hhh
[ Flow Control Set Header to… ]
The header (or more properly ‘CAN ID’) bytes
used for CAN Flow Control messages can be set using
this command. Only the right-most 11 bits of those
provided will be used - the most significant bit is
always removed. This command only affects Flow
Control mode 1.
FC SH hhhhhhhh
[ Flow Control Set Header to… ]
This command is used to set the header (or ‘CAN
ID’) bits for Flow Control responses with 29 bit CAN ID
systems. Since the 8 nibbles define 32 bits, only the
right-most 29 bits of those provided will be used - the
most significant three bits are always removed. This
command only affects Flow Control mode 1.
[ Flow Control Set Mode to h ]
This command sets how the ELM327 responds to
First Frame messages when automatic Flow Control
responses are enabled. The single digit provided can
either be ‘0’ (the default) for fully automatic responses,
‘1’ for completely user defined responses, or ‘2’ for
user defined data bytes in the response. Note that FC
modes 1 and 2 can only be enabled if you have
defined the needed data and possibly ID bytes. If you
have not, you will get an error. More complete details
and examples can be found in the Altering Flow
Control Messages section (page 46).
[ Forget Events ]
There are certain events which may change how
the ELM327 responds from that time onwards. One of
these is the occurrence of a fatal CAN error (ERR94),
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AT Command Descriptions (continued)
which blocks subsequent searching through CAN
protocols if PP 2A bit 5 is ‘1’. Normally, an event such
as this will affect all searches until the next power off
and on, but it can be ‘forgotten’ using software, with
the AT FE command.
Another example is an ‘LV RESET’ event which
will prevent searches through CAN protocols if PP 2A
bit 4 is ‘1’. It may also be forgotten with the AT FE
[ perform a Fast Initiation ]
One version of the Keyword protocol (the ELM327
protocol 5) uses what is known as a fast initiation
sequence to begin communications. Usually, this
sequence is performed when the first message needs
to be sent, and then the message is sent immediately
after. Some ECUs may need more time between the
two however, and having a separate initiation
command allows you to control this time. Simply send
AT FI, wait a little, then send the message. You may
need to experiment to get the right amount of delay.
Protocol 5 must be selected to use the AT FI
command, or an error will result.
IB 10
[ set the ISO Baud rate to 10400 ]
This command restores the ISO 9141-2 and
ISO 14230-4 baud rates to the default value of 10400.
IB 48
This command is used to change the baud rate
used for the ISO 9141-2 and ISO 14230-4 protocols
(numbers 3, 4, and 5) to 4800 baud, while relaxing
some of the requirements for the initiation byte
transfers. It may be useful for experimenting with some
vehicles. Normal (10,400 baud) operation may be
restored at any time with the IB 10 command.
IB 96
[ Headers off or on ]
These commands control whether or not the
additional (header) bytes of information are shown in
the responses from the vehicle. These are not
normally shown by the ELM327, but may be of interest
(especially if you receive multiple responses and wish
to determine what modules they were from).
Turning the headers on (with AT H1) actually
shows more than just the header bytes – you will see
the complete message as transmitted, including the
check-digits and PCI bytes, and possibly the CAN data
length code (DLC) if it has been enabled with PP 29 or
AT D1. The current version of this IC does not display
the CAN CRC code, nor the special J1850 IFR bytes
(which some protocols use to acknowledge receipt of a
[ Identify yourself ]
Issuing this command causes the chip to identify
itself, by printing the startup product ID string (currently
‘ELM327 v1.4b’). Software can use this to determine
exactly which integrated circuit it is talking to, without
having to reset the IC.
[ set the ISO Baud rate to 9600 ]
This command is used to change the baud rate
used for the ISO 9141-2 and ISO 14230-4 protocols
(numbers 3, 4, and 5) to 9600 baud, while relaxing
some of the requirements for the initiation byte
transfers. It may be useful for experimenting with some
vehicles. Normal (10,400 baud) operation may be
restored at any time with the IB 10 command.
IFR0, IFR1, and IFR2
H0 and H1
[ set the ISO Baud rate to 4800 ]
[ IFR control ]
The SAE J1850 protocol allows for an In-Frame
Response (IFR) byte to be sent after each message,
usually to acknowledge the correct receipt of that
message. The ELM327 automatically generates and
sends this byte for you by default, but you can override
this behaviour with this command.
The AT IFR0 command will disable the sending of
all IFRs, no matter what the header bytes require.
AT IFR2 is the opposite - it will cause an IFR byte to
always be sent, no matter what the header bytes say.
The AT IFR1 command is the default mode, with the
sending of IFRs determined by the ‘K’ bit of the first
header byte (for both PWM and VPW).
[ IFR from Header or Source ]
The value sent in the J1850 In-Frame Response
(IFR) byte is normally the same as the value sent as
the Source (or Tester) Address byte that was in the
header of the request. There may be occasions when
it is desirable to use some other value, however, and
this set of commands allows for this.
If you send AT IFR S, the ELM327 will use the
value defined as the Source Address (usually F1, but it
can be changed with PP 06), even if another value
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AT Command Descriptions (continued)
was sent in the Header bytes. This is not what is
normally required, and caution should be used when
using AT IFR S. AT IFR H restores the sending of the
IFR bytes to those provided in the Header, and is the
default setting.
[ read the IgnMon input level ]
This command reads the signal level at pin 15. It
assumes that the logic level is related to the ignition
voltage, so if the input is at a high level, the response
will be ‘ON’, and a low level will report ‘OFF’.
This feature is most useful if you wish to perform
the power control functions using your own software. If
you disable the Low Power automatic response to a
low input on this pin (by setting bit 2 of PP 0E to 0),
then pin 15 will function as the RTS input. A low level
on the input will not turn the power off, but it will
interrupt any OBD activity that is in progress. All you
need to do is detect the ‘STOPPED’ message that is
sent when the ELM327 is interrupted, and then check
the level at pin 15 using AT IGN. If it is found to be
OFF, you can perform an orderly shutdown yourself.
IIA hh
[ set the ISO Init Address to hh ]
The ISO 9141-2 and ISO 14230-4 standards state
that when beginning a session with an ECU, the
initiation sequence is to be directed to a specific
address ($33). If you wish to experiment by directing
the slow five baud sequence to another address, it is
done with this command. For example, if you prefer
that the initiation be performed with the ECU at
address $7A, then simply send:
and the ELM327 will use that address when called to
do so (protocols 3 or 4). The full eight bit value is used
exactly as provided – no changes are made to it (ie no
adding of parity bits, etc.)
Note that setting this value does not affect any
address values used in the header bytes. The ISO init
address is restored to $33 whenever the defaults, or
the ELM327, are reset.
example, to send a request for the engine temperature
(PGN 00FEEE), the data bytes are actually sent in the
reverse order (ie EE FE 00), and the ELM327 would
normally expect you to provide the data in that order
for passing on to the vehicle.
When experimenting, this constant need for byte
reversals can be quite confusing, so we have defined
an ELM format that reverses the bytes for you. When
the J1939 ELM (JE) format is enabled, and you have a
J1939 protocol selected, and you provide three data
bytes to the ELM327, it will reverse the order for you
before sending them to the ECU. To request the
engine temperature PGN, you would send 00 FE EE
(and not EE FE 00). The ‘JE’ type of automatic
formatting is enabled by default.
JHF0 and JHF1 [ J1939 Header Formatting off or on ]
When printing responses, the ELM327 normally
formats the J1939 ID (ie Header) bits in such a way as
to isolate the priority bits and group all the PGN
information, while keeping the source address byte
separate. If you prefer to see the ID information as four
separate bytes (which a lot of the J1939 software
seems to do), then simply turn off the formatting with
JHF0. The CAF0 command has the same effect (and
overrides the JHF setting), but also affects other
formatting. The default setting is JHF1.
[ enables the J1939 SAE data format ]
The AT JS command disables the automatic byte
reordering that the JE command performs for you. If
you wish to send data bytes to the J1939 vehicle
without any manipulation of the byte order, then select
JS formatting.
Using the above example for engine temperature
(PGN 00FEEE) with the data format set to JS, you
must send the bytes to the ELM327 as EE FE 00 (this
is also known as little-endian byte ordering).
The JS type of data formatting is off by default, but
was the only type of data formatting provided by the
ELM327 v1.2. If you are switching from version 1.2 of
the IC, take note of this difference.
[ enables the J1939 ELM data format ]
The J1939 standard requires that PGN requests
be sent with the byte order reversed from the standard
‘left-to-right’ order, which many of us would expect. For
[ J1939 Timer Multiplier to 1 ]
This command sets the J1939 AT ST time
multiplier to 1, reversing any changes made by JTM5.
JTM1 is the default setting. It has no effect for nonJ1939 protocols.
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AT Command Descriptions (continued)
[ J1939 Timer Multiplier to 5 ]
When using a J1939 protocol, it is occasionally
useful to be able to set the AT ST time to values
longer than one second. The JTM5 command will
multiply the AT ST setting by a factor of 5, in order to
provide longer times for the J1939 protocols (only). By
default, this multiplier is off.
terminal program, but off if using a custom computer
interface (as the extra characters transmitted will only
serve to slow the communications down). The default
setting is determined by the voltage at pin 7 during
power on (or reset). If the level is high, then linefeeds
are on by default; otherwise they will be off.
[ display the Key Words ]
When the ISO 9141-2 and ISO 14230-4 protocols
are initialized, two special bytes (key words) are
passed to the ELM327 (the values are used internally
to determine whether a particular protocol variation
can be supported by the ELM327). If you wish to see
what the value of these bytes were, simply send the
AT KW command.
KW0 and KW1
[ Key Word checks off or on ]
The ELM327 looks for specific bytes (called key
words) to be sent to it during the ISO 9141-2 and
ISO14230-4 initiation sequences. If the bytes are not
found, the initiation is said to have failed (you might
see ‘UNABLE TO CONNECT’ or perhaps ‘BUS INIT:
...ERROR’). This might occur if you are trying to
connect to a non-OBD compliant ECU, or perhaps to
an older one.
If you wish to experiment with non-standard
systems, you may have to tell the ELM327 to perform
the initiation sequence, but ignore the contents of the
bytes that are sent and received. To do this, send:
After turning keyword checking off, the ELM327
will still require the two key word bytes in the
response, but will not look at the actual values of the
bytes. It will also send an acknowledgement to the
ECU, and will wait for the final response from it (but
will not stop and report an error if none is received).
This may allow you to make a connection in an
otherwise ‘impossible’ situation. Normal behaviour can
be returned with AT KW1, which is the default setting.
L0 and L1
[ Linefeeds off or on ]
This option controls the sending of linefeed
characters after each carriage return character. For
AT L1, linefeeds will be generated after every carriage
return character, and for AT L0, they will be off. Users
will generally wish to have this option on if using a
[ go to the Low Power mode ]
This command causes the ELM327 to shut off all
but ‘essential services’ in order to reduce the power
consumption to a minimum. The ELM327 will respond
with an ‘OK’ (but no carriage return) and then, one
second later, will change the state of the PwrCtrl
output (pin 16) and will enter the low power (standby)
mode. The IC can be brought back to normal operation
through a character received at the RS232 input or a
rising edge at the IgnMon (pin 15) input, in addition to
the usual methods of resetting the IC (power off then
on, a low on pin 1, or a brownout). See the Power
Control section (page 62) for more information.
M0 and M1
[ Memory off or on ]
The ELM327 has internal ‘non-volatile’ memory
that is capable of remembering the last protocol used,
even after the power is turned off. This can be
convenient if the IC is often used for one particular
protocol, as that will be the first one attempted when
next powered on. To enable this memory function, it is
necessary to either use an AT command to select the
M1 option, or to have chosen ‘memory on’ as the
default power on mode (by connecting pin 5 of the
ELM327 to a high logic level).
When the memory function is enabled, each time
that the ELM327 finds a valid OBD protocol, that
protocol will be memorized (stored) and will become
the new default. If the memory function is not enabled,
protocols found during a session will not be
memorized, and the ELM327 will always start at power
up using the same (last saved) protocol.
If the ELM327 is to be used in an environment
where the protocol is constantly changing, it would
likely be best to turn the memory function off, and
issue an AT SP 0 command once. The SP 0 command
tells the ELM327 to start in an 'Automatic' protocol
search mode, which is the most useful for an unknown
environment. ICs come from the factory set to this
mode. If, however, you have only one vehicle that you
regularly connect to, storing that vehicle’s protocol as
the default would make the most sense.
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AT Command Descriptions (continued)
The default setting for the memory function is
determined by the voltage level at pin 5 during power
up (or system reset). If it is connected to a high level
(VDD), then the memory function will be on by default.
If pin 5 is connected to a low level, the memory saving
will be off by default.
[ Monitor All messages ]
This command places the ELM327 into a bus
monitoring mode, in which it continually monitors for
(and displays) all messages that it sees on the OBD
bus. It is a quiet monitor, not sending In Frame
Responses for J1850 systems, Acknowledges for CAN
systems, or Wakeup (‘keep-alive’) messages for the
ISO 9141 and ISO 14230 protocols. Monitoring will
continue until it is stopped by activity on the RS232
input, or the RTS pin.
To stop the monitoring, simply send any single
character to the ELM327, then wait for it to respond
with a prompt character (‘>’), or a low level output on
the Busy pin. (Setting the RTS input to a low level will
interrupt the device as well.) Waiting for the prompt is
necessary as the response time varies depending on
what the IC was doing when it was interrupted. If for
instance it is in the middle of printing a line, it will first
complete that line then return to the command state,
issuing the prompt character. If it were simply waiting
for input, it would return immediately. Note that the
character which stops the monitoring will always be
discarded, and will not affect subsequent commands.
Beginning with v1.3 of this IC, all messages will be
printed as found, even if the CAN auto formatting is on
(CAF1). The previous version of this IC (v1.2) did not
display some illegal CAN messages if the automatic
formatting was on, but now all messages received are
displayed, and if the data format does not appear to be
correct, then ‘<DATA ERROR’ will be shown beside
the data.
If this command is used with CAN protocols, and if
the CAN filter and/or mask were previously set (with
CF, CM or CRA), then the MA command will be
affected by the settings. For example, if the receive
address had been set previously with CRA 4B0, then
the AT MA command would only be able to ‘see’
messages with an ID of 4B0. This may not be what is
desired - you may want to reset the masks and filters
(with AT AR) first.
All of the monitoring commands (MA, MR and MT)
operate by closing the current protocol (an AT PC is
executed internally), then configuring the IC for silent
monitoring of the data (no wakeup messages, IFRs or
CAN acknowledges are sent by the ELM327). When
the next OBD command is to be transmitted, the
protocol will again be initialized, and you may see
messages stating this. ‘SEARCHING...’ may also be
seen, depending on what changes were made while
MP hhhh
[ Monitor for PGN hhhh ]
The AT MA, MR and MT commands are quite
useful for when you wish to monitor for a specific byte
in the header of a typical OBD message. For the SAE
J1939 Protocol, however, it is often desirable to
monitor for the multi-byte Parameter Group Numbers
(or PGNs), which can appear in either the header, or
the data bytes. The MP command is a special J1939
only command that is used to look for responses to a
particular PGN request.
Note that this MP command provides no means to
set the first two digits of the requested PGN, and they
are always assumed to be 00. For example, the DM2
PGN has an assigned value of 00FECB (see SAE
J1939-73). To monitor for DM2 messages, you would
issue AT MP FECB, eliminating the 00, since the
MP hhhh command always assumes that the PGN is
preceded by two zeros.
This command is only available when a CAN
Protocol (A, B, or C) has been selected for SAE J1939
formatting. It returns an error if attempted under any
other conditions. Note also that this version of the
ELM327 only displays responses that match the
criteria, not the requests that are asking for the PGN
MP hhhh n
[ Monitor for PGN, get n messages ]
This is very similar to the above command, but
adds the ability to set the number of messages that
should be fetched before the ELM327 automatically
stops monitoring and prints a prompt character. The
value ‘n’ may be any single hex digit.
MP hhhhhh
[ Monitor for PGN hhhhhh ]
This command is very similar to the MP hhhh
command, but it extends the number of bytes provided
by one, so that there is complete control over the PGN
definition (it does not make the assumption that the
Data Page bit is 0, as the MP hhhh command does).
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AT Command Descriptions (continued)
This allows for future expansion, should additional
PGNs be defined with the Data Page bit set. Note that
only the Data Page bit is relevant in the extra byte the other bits are ignored.
MP hhhhhh n
[ Monitor for PGN, get n messages ]
This is very similar to the previous command, but it
adds the ability to set the number of messages that
should be fetched before the ELM327 automatically
stops monitoring and prints a prompt character. The
value ‘n’ may be any single hex digit.
MR hh
[ Monitor for Receiver hh ]
This command is very similar to the AT MA
command except that it will only display messages that
were sent to the hex address given by hh. These are
messages which are found to have the value hh in the
second byte of a traditional three byte OBD header, in
bits 8 to 15 of a 29 bit CAN ID, or in bits 8 to 10 of an
11 bit CAN ID. Any single RS232 character aborts the
monitoring, as with the MA command.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MR
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
CRA 4B0, and then you send MR 02, the 02 will
replace the 4, and the CAN masks/filters will only allow
IDs that are equal to 2B0. This is often not what is
desired - you may want to reset the masks and filters
(with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
MT hh
[ Monitor for Transmitter hh ]
This command is also very similar to the AT MA
command, except that it will only display messages
that were sent by the transmitter with the hex address
given by hh. These are messages which are found to
have that value in the third byte of a traditional three
byte OBD header, or in bits 0 to 7 for CAN IDs. As with
the MA and MR monitoring modes, any RS232 activity
(single character) aborts the monitoring.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MT
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
CRA 4B0, and then you send MT 20, the 20 will
replace the B0, and the CAN masks/filters will only
allow IDs that are equal to 420. This is often not what
is desired - you may want to reset the masks and
filters (with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
[ Normal Length messages ]
Setting the NL mode on forces all sends and
receives to be limited to the standard seven data bytes
in length, similar to the other ELM32x OBD ICs. To
allow longer messages, use the AL command.
Beginning with v1.2, the ELM327 does not require
a change to AL to allow longer message lengths for
the KWP protocols to be received (as determined by
the header length values). You can simply leave the IC
set to the default setting of NL, and all of the received
bytes will be shown.
PB xx yy
[ set Protocol B parameters ]
This command allows you to change the protocol
B (USER1) options and baud rate without having to
change the associated Programmable Parameters.
This allows for easier testing, and program control.
To use this feature, simply set xx to the value for
PP 2C, and yy to the value for PP 2D, and issue the
command. The next time that the protocol is initialized
it will use these values. For example, assume that you
wish to try monitoring a system that uses 11 bit CAN at
33.3 kbps. If you do not want any special formatting,
this means a value of 11000000 or C0 for PP 2C, and
15 decimal or 0F hex for PP 2D. Send these values to
the ELM327 in one command:
>AT PB C0 0F
then monitor:
if you see CAN ERRORs, and realize that you wanted
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AT Command Descriptions (continued)
an 83.3 kbps baud rate, close the protocol, and then
send the new values:
>AT PB C0 06
the IC to perform a normal startup. Note that a reset of
the PPs occurs quite quickly – if you are holding the
jumper on for more than a few seconds and do not see
the RS232 receive light flashing, remove the jumper
and try again, as there may be a problem with your
connection. This feature is only available beginning
with v1.2, and is not a provided with any earlier
versions of the ELM327 IC.
Values passed in this way do not affect those that
are stored in the 2C and 2D Programmable
Parameters, and are lost if the ELM327 is reset. If you
want to make your settings persist over power cycles,
then you should store them in the Programmable
Parameter memory (don’t forget that there are two –
USER1 and USER2.
[ Protocol Close ]
There may be occasions where it is desirable to
stop (deactivate) a protocol. Perhaps you are not using
the automatic protocol finding, and wish to manually
activate and deactivate protocols. Perhaps you wish to
stop the sending of idle (wakeup) messages, or have
another reason. The PC command is used in these
cases to force a protocol to close.
[ turn Prog. Parameter hh OFF ]
This command disables Programmable Parameter
number hh. Any value assigned using the PP hh SV
command will no longer be used, and the factory
default setting will once again be in effect. The actual
time when the new value for this parameter becomes
effective is determined by its type. Refer to the
Programmable Parameters section (page 55) for more
information on the types.
Note that ‘PP FF OFF’ is a special command that
disables all of the Programmable Parameters, as if you
had entered PP OFF for every possible one.
It is possible to alter some of the Programmable
Parameters so that it may be difficult, or even
impossible, to communicate with the ELM327. If this
occurs, there is a hardware means of resetting all of
the Programmable Parameters at once. Connect a
jumper from circuit common to pin 28, holding it there
while powering up the ELM327 circuit. Hold it in
position until you see the RS232 Receive LED begin to
flash (which indicates that all of the PPs have been
turned off). At this point, remove the jumper to allow
PP hh ON
[ turn Programmable Parameter hh ON ]
This command enables Programmable Parameter
number hh. Once enabled, any value assigned using
the PP hh SV command will be used where the factory
default value was before. (All of the programmable
parameter values are set to their default values at the
factory, so enabling a programmable parameter before
assigning a value to it will not cause problems.) The
actual time when the value for this parameter becomes
effective is determined by its type. Refer to the
Programmable Parameters section (page 55) for more
information on the types.
Note that ‘PP FF ON’ is a special command that
enables all of the Programmable Parameters at the
same time.
PP hh SV yy [ Prog. Param. hh: Set the Value to yy ]
A value is assigned to a Programmable Parameter
using this command. The system will not be able to
use this new value until the Programmable Parameter
has been enabled, with PP hh ON.
[ Programmable Parameter Summary ]
The complete range of current Programmable
Parameters are displayed with this command (even
those not yet implemented). Each is shown as a PP
number followed by a colon and the value that is
assigned to it. This is followed by a single digit – either
‘N’ or ‘F’ to show that it is ON (enabled), or OFF
(disabled), respectively. See the Programmable
Parameters section for a more complete discussion.
R0 and R1
[ Responses off or on ]
These commands control the ELM327’s automatic
receive (and display) of the messages returned by the
vehicle. If responses have been turned off, the IC will
not wait for a reply from the vehicle after sending a
request, and will return immediately to wait for the next
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AT Command Descriptions (continued)
RS232 command (the ELM327 does not print anything
to say that the send was successful, but you will see a
message if it was not).
R0 may be useful to send commands blindly when
using the IC for a non-OBD network application, or
when simulating an ECU in a learning environment. It
is not recommended that this option used for normal
OBD communications, however, as the vehicle may
have difficulty if it is expecting an acknowledgement
and never receives one.
An R0 setting will always override any ‘number of
responses digit’ that is provided with an OBD request.
The default setting is R1, or responses on.
make any assumptions as to what format a response
may have), so adjustments may need to be made to
the mask and filter. This command must be used with
an active CAN protocol (one that has been sending
and receiving messages), as it can not initiate a
protocol search. Note that the CAF1 setting normally
eliminates the display of all RTRs, so if you are
monitoring messages and want to see the RTRs, you
will have to turn off formatting, or else turn the headers
The ELM327 treats an RTR just like any other
message sent, and will wait for a response from the
vehicle (unless AT R0 has been chosen).
RA hh
[ set the Receive Address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM327 will
respond. Issuing this command will turn off the AR
mode, and force the IC to only accept responses
addressed to hh. Use caution with this setting, as
depending on what you set it to, you may end up
accepting (ie. acknowledging with an IFR) a message
that was actually meant for another module. To turn off
the RA filtering, simply send AT AR.
This command is not very effective for use with the
CAN protocols, as it only monitors for one portion of
the ID bits, and that is not likely enough for most CAN
applications - the CRA command may be a better
choice. Also, this command has no effect on the
addresses used by the J1939 protocols, as the J1939
routines derive them from the header values, as
required by the SAE standard.
The RA command is exactly the same as the SR
command, and can be used interchangeably. Note that
CAN Extended Addressing does not use this value - it
uses the one set by the AT TA command.
[ Read the Data in the user memory ]
The byte value stored with the SD command is
retrieved with this command. There is only one
memory location, so no address is required.
[ send an RTR message ]
This command causes a special ‘Remote Frame’
CAN message to be sent. This type of message has
no data bytes, and has its Remote Transmission
Request (RTR) bit set. The headers and filters will
remain as previously set (ie the ELM327 does not
[ Read the input Voltage ]
This initiates the reading of the voltage present at
pin 2, and the conversion of it to a decimal voltage. By
default, it is assumed that the input is connected to the
voltage to be measured through a 47KΩ and 10KΩ
resistor divider (with the 10KΩ connected from pin 2 to
Vss), and that the ELM327 supply is a nominal 5V.
This will allow for the measurement of input voltages
up to about 28V, with an uncalibrated accuracy of
typically about 2%.
S0 and S1
[ printing of Spaces off or on ]
These commands control whether or not space
characters are inserted in the ECU response.
The ELM327 normally reports ECU responses as
a series of hex characters that are separated by space
characters (to improve readability), but messages can
be transferred much more quickly if every third byte
(the space) is removed. While this makes the message
less readable for humans, it can provide significant
improvements for computer processing of the data. By
default, spaces are on (S1), and space characters are
inserted in every response.
SD hh
[ Save Data byte hh ]
The ELM327 is able to save one byte of
information for you in a special nonvolatile memory
location, which is able to retain its contents even if the
power is turned off. Simply provide the byte to be
stored, then retrieve it later with the read data (AT RD)
command. This location is ideal for storing user
preferences, unit ids, occurrence counts, or other
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AT Command Descriptions (continued)
SH xyz
[ Set the Header to 00 0x yz ]
Entering CAN 11 bit ID words (headers) normally
requires that extra leading zeros be added (eg. AT SH
00 07 DF), but this command simplifies doing so. The
AT SH xyz command accepts a three digit argument,
takes only the right-most 11 bits from that, adds
leading zeros, and stores the result in the header
storage locations for you. As an example, AT SH 7DF
is a valid command, and is quite useful for working
with 11 bit CAN systems. It actually results in the
header bytes being stored internally as 00 07 DF.
SH xx yy zz
[ Set the Header to xx yy zz ]
This command allows the user to manually control
the values that are sent as the three header bytes in a
message. These bytes are normally assigned values
for you (and are not required to be adjusted), but there
may be occasions when it is desirable to change them
(particularly if experimenting with physical addressing).
If experimenting, it is not necessary but may be better
to set the headers after a protocol is active. That way,
wakeup messages, etc. that get set on protocol
activation will use the default values.
The header bytes are defined with hexadecimal
digits - xx will be used for the first or priority/type byte,
yy will be used for the second or receiver/target byte,
and zz will be used for the third or transmitter/source
byte. These remain in effect until set again, or until
restored to their default values with the D, WS, or Z
If new values for header bytes are set before the
vehicle protocol has been determined, and if the
search is not set for fully automatic (ie other than
protocol 0), these new values will be used for the
header bytes of the first request to the vehicle. If that
first request should fail to obtain a response, and if the
automatic search is enabled, the ELM327 will then
continue to search for a protocol using default values
for the header bytes. Once a valid protocol is found,
the header bytes will revert to the values assigned with
the AT SH command.
This command is used to assign all header bytes,
whether they are for a J1850, ISO 9141, ISO 14230, or
a CAN system. The CAN systems will use these three
bytes to fill bits 0 to 23 of the ID word (for a 29 bit ID),
or will use only the rightmost 11 bits for an 11 bit CAN
ID (and any extra bits assigned will be ignored). The
additional 5 bits needed for a 29 bit system are set
with the AT CP command.
If assigning header values for the KWP protocols
(4 and 5), care must be taken when setting the first
header byte (xx) value. The ELM327 will always insert
the number of data bytes for you, but how it is done
depends on the values that you assign to this byte. If
the second digit of this first header byte is anything
other than 0 (zero), the ELM327 assumes that you
wish to have the length value inserted in that first byte
when sending. In other words, providing a length value
in the first header byte tells the ELM327 that you wish
to use a traditional 3 byte header, where the length is
stored in the first byte of the header.
If you provide a value of 0 for the second digit of
the first header byte, the ELM327 will assume that you
wish that value to remain as 0, and that you want to
have a fourth header (length) byte inserted into the
message. This is contrary to the ISO 14230-4 OBD
standard, but it is in use by many KWP2000 systems
for (non-OBD) data transfer, so may be useful when
experimenting. Support for 4 byte KWP headers was
added with v1.2 of the ELM327 IC, and is not available
in previous versions.
[ perform a Slow Initiation ]
Protocols 3 and 4 use what is sometimes called a
5 baud, or slow initiation sequence in order to begin
communications. Usually, the sequence is performed
when the first message needs to be sent, and then the
message is sent immediately after. Some ECUs may
need more time between the two however, and having
a separate initiation command allows you to control
this time. Simply send AT SI, wait a little, then send
the message. You may need to experiment a little to
get the right amount of delay. Protocol 3 or 4 must be
selected to use the AT SI command, or an error will
SP h
[ Set Protocol to h ]
This command is used to set the ELM327 for
operation using the protocol specified by 'h', and to
also save it as the new default. Note that the protocol
will be saved no matter what the AT M0/M1 setting is.
The ELM327 supports 12 different protocols (two
can be user-defined). They are:
0 - Automatic
1 - SAE J1850 PWM (41.6 kbaud)
2 - SAE J1850 VPW (10.4 kbaud)
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AT Command Descriptions (continued)
3 - ISO 9141-2 (5 baud init, 10.4 kbaud)
4 - ISO 14230-4 KWP (5 baud init, 10.4 kbaud)
5 - ISO 14230-4 KWP (fast init, 10.4 kbaud)
6 - ISO 15765-4 CAN (11 bit ID, 500 kbaud)
7 - ISO 15765-4 CAN (29 bit ID, 500 kbaud)
8 - ISO 15765-4 CAN (11 bit ID, 250 kbaud)
9 - ISO 15765-4 CAN (29 bit ID, 250 kbaud)
A - SAE J1939 CAN (29 bit ID, 250* kbaud)
B - USER1 CAN (11* bit ID, 125* kbaud)
C - USER2 CAN (11* bit ID, 50* kbaud)
* default settings (user adjustable)
The first protocol shown (0) is a convenient way of
telling the ELM327 that the vehicle’s protocol is not
known, and that it should perform a search. It causes
the ELM327 to try all protocols if necessary, looking for
one that can be initiated correctly. When a valid
protocol is found, and the memory function is enabled,
that protocol will then be remembered, and will
become the new default setting. When saved like this,
the automatic mode searching will still be enabled, and
the next time the ELM327 fails to connect to the saved
protocol, it will again search all protocols for another
valid one. Note that some vehicles respond to more
than one protocol - during a search, you may see more
than one type of response.
ELM327 users often use the AT SP 0 command to
reset the search protocol before starting (or restarting)
a connection. This works well, but as with any Set
Protocol command, it involves a write to EEPROM,
and an unnecessary delay (of about 30 msec) while
the write occurs. Beginning with v1.3 of the ELM327, a
write to EEPROM will no longer be performed for an
SP 0 (or an SP A0, or SP 0A) command, but the
command will still reset the protocol to 0 for you. If you
really want to change what is stored in the internal
EEPROM, you must now use the new AT SP 00
If another protocol (other than 0) is selected with
this command (eg. AT SP 3), that protocol will become
the default, and will be the only protocol used by the
ELM327. Failure to initiate a connection in this
situation will result in a response such as ‘BUS INIT:
...ERROR’, and no other protocols will be attempted.
This is a useful setting if you know that your vehicle(s)
only require the one protocol, but also one that can
cause a lot of problems if you do not understand it .
[ Set Protocol to Auto, h ]
This variation of the SP command allows you to
choose a starting (default) protocol, while still retaining
the ability to automatically search for a valid protocol
on a failure to connect. For example, if your vehicle is
ISO 9141-2, but you want to occasionally use the
ELM327 circuit on other vehicles, you might use the
AT SP A3 command, so that the first protocol tried will
then be yours (3), but it will also automatically search
for other protocols. Don't forget to disable the memory
function if doing this, or each new protocol detected
will become your new default.
SP Ah will save the protocol information even if
the memory option is off, except for SP A0 and SP 0A
which as of v1.3 no longer performs the write (if you
need to change the EEPROM, use SP 00). Note that
the ‘A’ can come before or after the h, so AT SP A3
can also be entered as AT SP 3A.
SR hh
[Set the Receive address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM327 will
respond. Issuing this command will turn off the AR
mode, and force the IC to only accept responses
addressed to hh. Use caution with this setting, as
depending on what you set it to, you may accept a
message that was actually meant for another module,
possibly sending an IFR when you should not. To turn
off the SR filtering, simply send AT AR.
This command has limited use with CAN, as it only
monitors one byte of the ID bits, and that is not likely
selective enough for most CAN applications (the CRA
command may be a better choice). Also, the command
has no effect on the addresses used by the J1939
protocols, as the J1939 routines set their own receive
addresses based on the ID bit (header) values.
This SR command is exactly the same as the RA
command, and can be used interchangeably with it.
Note that CAN Extended Addressing does not use this
value - it uses the one set by the AT TA command.
[ use the Standard Sequence for searches ]
SAE standard J1978 specifies a protocol search
order that scan tools should use. It follows the number
order that we have assigned to the ELM327 protocols.
In order to provide a faster search, the ELM327 does
not normally follow this order, but it will if you
command it to with AT SS.
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AT Command Descriptions (continued)
ST hh
[ Set Timeout to hh ]
After sending a request, the ELM327 waits a
preset time for a response before it can declare that
there was ‘NO DATA’ received from the vehicle. The
same timer setting is also used after a response has
been received, while waiting to see if more are
coming. The AT ST command allows this timer to be
adjusted, in increments of 4 msec (or 20 msec if in the
J1939 protocol, with JTM5 selected).
When Adaptive Timing is enabled, the AT ST time
sets the maximum time that is to be allowed, even if
the adaptive algorithm determines that the setting
should be longer. In most circumstances, it is best to
simply leave the AT ST time at the default setting, and
let the adaptive timing algorithm determine what to use
for the timeout.
The ST timer is set to 32 by default (giving a time
of approximately 200 msec), but this value can be
adjusted by changing PP 03. Note that a value of 00
does not result in a time of 0 msec – it will restore the
timer to the default value.
SW hh
[ Set Wakeup to hh ]
Once a data connection has been established,
some protocols require that there be data flow every
few seconds, just so that the ECU knows to maintain
the communications path open. If the messages do not
appear, the ECU will assume that you are finished,
and will close the channel. The connection will need to
be initialized again to reestablish communications.
The ELM327 will automatically generate periodic
messages, as required, in order to maintain a
connection. The replies to these messages are always
ignored, and are not visible to the user. (Currently,
only protocols 3, 4, and 5 support these messages nothing is available for CAN.)
The time interval between these periodic ‘wakeup’
messages can be adjusted in 20 msec increments
using the AT SW hh command, where hh is any
hexadecimal value from 00 to FF. The maximum
possible time delay of just over 5 seconds results
when a value of FF (decimal 255) is used. The default
setting provides a nominal delay of 3 seconds between
Note that the value 00 (zero) is treated as a very
special case, and must be used with caution, as it will
stop all periodic messages. This way of stopping the
messages while keeping the rest of the protocol
functioning normally, is for experimenters, and is not
intended to be used regularly. Issuing AT SW 00 will
not change a prior setting for the time between
wakeup messages, if the protocol is re-initialized.
TA hh
[ set the Tester Address to hh ]
This command is used to change the current
tester (ie. scan tool) address that is used in the
headers, periodic messages, filters, etc. The ELM327
normally uses the value that is stored in PP 06 for this,
but the TA command allows you to temporarily
override that value.
Sending AT TA will affect all protocols, including
J1939. This provides a convenient means to change
the J1939 address from the default value of F9,
without affecting other settings.
Although this command may appear to work ‘on
the fly’, it is not recommended that you try to change
this address after a protocol is active, as the results
may be unpredictable.
TP h
[ Try Protocol h ]
This command is identical to the SP command,
except that the protocol that you select is not
immediately saved in internal memory, so does not
change the default setting. Note that if the memory
function is enabled (AT M1), and this new protocol that
you are trying is found to be valid, that protocol will
then be stored in memory as the new default.
[ Try Protocol h with Auto ]
This command is very similar to the AT TP
command above, except that if the protocol that is tried
should fail to initialize, the ELM327 will then
automatically sequence through the other protocols,
attempting to connect to one of them.
V0 and V1
[ Variable data lengths off or on ]
These commands modify the current CAN protocol
settings to allow the sending of variable data length
messages, just as bit 6 of PP 2C and PP 2E do for
protocols B and C. This allows experimenting with
variable data length messages for any of the CAN
protocols (not just B and C). The V1 command will
always override any protocol setting, and force a
variable data length message. The default setting is
V0, providing data lengths as determined by the
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AT Command Descriptions (continued)
WM [1 to 6 bytes]
[ set Wakeup Message to… ]
This command allows the user to override the
default settings for the wakeup messages (sometimes
known as the ‘periodic idle’ messages). Simply provide
the message that you wish to have sent (typically three
header bytes and one to three data bytes), and the
ELM327 will then send them as required, at the rate
determined by the AT SW setting. Note that you do not
have to add a checksum byte to the data – the
ELM327 calculates the value and adds it for you.
[ Warm Start ]
This command causes the ELM327 to perform a
complete reset which is very similar to the AT Z
command, but does not include the power on LED
test. Users may find this a convenient way to quickly
‘start over’ without having the extra delay of the AT Z
If using variable RS232 baud rates (ie AT BRD
commands), it is preferred that you reset the IC using
this command rather than AT Z, as AT WS will not
affect the chosen RS232 baud rate, and AT Z will.
[ reset all ]
This command causes the chip to perform a
complete reset as if power were cycled off and then on
again. All settings are returned to their default values,
and the chip will be put into the idle state, waiting for
characters on the RS232 bus. Any baud rate that was
set with the AT BRD command will be lost, and the
ELM327 will return to the default baud rate setting.
[ display the device description ]
This command displays the device description
string. The default text is ‘OBDII to RS232 Interpreter’.
[ display the device identifier ]
A device identifier string that was recorded with
the @3 command is displayed with the @2 command.
All 12 characters and a terminating carriage return will
be sent in response, if they have been defined. If no
identifier has been set, the @2 command returns an
error response (‘?’). The identifier may be useful for
storing product codes, production dates, serial
numbers, or other such codes.
@3 cccccccccccc
[ store the device identifier ]
This command is used to set the device identifier
code. Exactly 12 characters must be sent, and once
written to memory, they can not be changed (ie you
may only use the @3 command one time). The
characters sent must be printable (ascii character
values 00x21 to 0x5F inclusive).
If you are developing software to write device
identifiers, you may be interested in the ELM328 IC, as
it allows multiple writes using the @3 command (but it
can not send OBD messages).
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Reading the Battery Voltage
Before learning the OBD Commands, we will show
an example of how to use an AT Command. We will
assume that you have built (or purchased) a circuit
which is similar to that of Figure 9 in the Example
Applications section (page 67). This circuit provides a
connection to read the vehicle’s battery voltage, which
many will find very useful.
If you look in the AT Command list, you will see
there is one command that is listed as RV [Read the
input Voltage]. This is the command which you will
need to use. First, be sure that the prompt character is
shown (that is the ‘>’ character), then simply enter ‘AT’
followed by RV, and press return (or enter):
Note that we used upper case characters for this
request, but it was not required, as the ELM327 will
accept upper case (AT RV) as well as lower case
(at rv) or any combination of these (At rV). It does not
matter if you insert space characters (‘ ’) within the
message either, as they are ignored by the ELM327.
A typical response to this command will show a
voltage reading, followed by another prompt character:
the CV value, as the ELM327 knows that it should be
between the second and the third digits.
At this point, the internal calibration values have
been changed (ie. written to EEPROM), and the
ELM327 now knows that the voltage at the input is
actually 12.47V. To verify that the changes have taken
place, simply read the voltage again:
The ELM327 always rounds off the measurement
to one decimal place, so the 12.47V actually appears
as 12.5V (but the second decimal place is maintained
internally for accuracy and is used in the calculations).
The ELM327 may be calibrated with any reference
voltage that you have available, but note that the CV
command always expects to receive four characters
representing the voltage at the input. If you had used a
9V battery for your reference, and it is actually 9.32V,
then you must add a leading zero to the actual voltage
when calibrating the IC:
>AT CV 0932
The accuracy of this reading depends on several
factors. As shipped from the factory, the ELM327
voltage reading circuitry will typically be accurate to
about 2%. For many, this is all that is needed. Some
people may want to calibrate the circuitry for more
accurate readings, however, so we have provided a
special ‘Calibrate Voltage’ command for this.
To change the internal calibration constants, you
will need to know the actual battery voltage to more
accuracy than the ELM327 shows. Many quality digital
multimeters can do this, but you should verify the
accuracy before making a change.
Let us assume that you have connected your
accurate multimeter, and you find that it reads 12.47V.
The ELM327 is a little high at 12.6V, and you would
like it to read the same as your meter. Simply calibrate
the ELM327 to the measured voltage using the CV
If you should get into trouble with this command
(for example, if you set calibration values to something
arbitrary and do not have a voltmeter on hand to
provide accurate values), you can restore the settings
to the original (factory) values with the CV 0000
command. Simply send:
>AT CV 0000
The other AT Commands are used in the same
manner. Simply type the letters A and T, then follow
with the command you want to send and any
arguments that are required. Then press return (or
enter, depending on your keyboard). Remember - you
can always insert space characters as often as you
wish if it improves the readability for you, as they are
ignored by the ELM327.
>AT CV 1247
Note that you should not provide a decimal point in
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OBD Commands
If the bytes that you send to the ELM327 do not
begin with the letters ‘A’ and ‘T’, they are assumed to
be OBD commands for the vehicle. Each pair of ASCII
bytes will be tested to ensure that they are valid
hexadecimal digits, and will then be combined into
data bytes for transmitting to the vehicle.
OBD commands are actually sent to the vehicle
embedded in a data packet. Most standards require
that three header bytes and an error checksum byte
be included with every OBD message, and the
ELM327 adds these extra bytes to your command
bytes for you. The initial (default) values for these
extra bytes are usually appropriate for most requests,
but if you wish to change them, there is a mechanism
to do so (see the ‘Setting the Headers’ section).
Most OBD commands are only one or two bytes in
length, but some can be longer. The ELM327 will limit
the number of bytes that can be sent to the maximum
number allowed by the standards (usually seven bytes
or 14 hexadecimal digits). Attempts to send more
bytes will result in an error – the entire command is
then ignored and a single question mark printed.
Hexadecimal digits are used for all of the data
exchange with the ELM327 because it is the data
format used most often in the OBD standards. Most
mode request listings use hexadecimal notation, and it
is the format most frequently used when results are
shown. With a little practice, it should not be very
difficult to deal in hex numbers, but some people may
want to use a table such as Figure 1, or keep a
calculator nearby. Dealing with the hex digits can not
be avoided - eventually all users need to manipulate
the results in some way (combining bytes and dividing
by 4 to obtain rpm, dividing by 2 to obtain degrees of
advance, converting temperatures, etc.).
As an example of sending a command to the
vehicle, assume that A6 (or decimal 166) is the
command that is required to be sent. In this case, the
user would type the letter A, then the number 6, then
would press the return key. These three characters
would be sent to the ELM327 by way of the RS232
port. The ELM327 would store the characters as they
are received, and when the third character (the
carriage return) was received, would begin to assess
the other two. It would see that they are both valid hex
digits, and would convert them to a one byte value (the
decimal value is 166). The header bytes and a
checksum byte would then be added, and a total of
five bytes would typically be sent to the vehicle. Note
that the carriage return character is only a signal to the
ELM327, and is not sent to the vehicle.
After sending the command, the ELM327 listens
on the OBD bus for replies, looking for ones that are
directed to it. If a message address matches, the
received bytes will be sent on the RS232 port to the
user, while messages received that do not have
matching addresses will be ignored (but are often still
available for viewing with the AT BD command).
The ELM327 will continue to wait for messages
addressed to it until there are none found in the time
that was set by the AT ST command. As long as
messages continue to be received, the ELM327 will
continue to reset this timer, and look for more. Note
that the IC will always respond to a request with some
reply, even if it is to say ‘NO DATA’ (meaning that
there were no messages found, or that some were
found but they did not match the receive criteria).
Figure 1. Hex to Decimal Conversion
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Talking to the Vehicle
The standards require that each OBD command or
request that is sent to the vehicle must adhere to a set
format. The first byte sent (known as the ‘mode’)
describes the type of data being requested, while the
second byte (and possibly a third or more) specifies
the actual information that is required. The bytes which
follow after the mode byte are known as the
‘parameter identification’ or PID number bytes. The
modes and PIDs are described in detail in documents
such as the SAE J1979, or ISO 15031-5 standards,
and may also be defined by the vehicle manufacturers.
The SAE J1979 standard currently defines ten
possible diagnostic test modes, which are:
- show current data
- show freeze frame data
- show diagnostic trouble codes
- clear trouble codes and stored values
- test results, oxygen sensors
- test results, non-continuously monitored
- show ‘pending’ trouble codes
- special control mode
- request vehicle information
- request permanent trouble codes
Vehicles are not required to support all of the
modes, and within modes, they are not required to
support all possible PIDs (some of the first OBDII
vehicles only supported a very small number of them).
Within each mode, PID 00 is reserved to show which
PIDs are supported by that mode. Mode 01, PID 00
must be supported by all vehicles, and can be
accessed as follows…
Ensure that your ELM327 interface is properly
connected to the vehicle, and powered. Most vehicles
will not respond without the ignition key in the ON
position, so turn the ignition to on, but do not start the
engine. If you have been experimenting, the state of
your interface may be unknown, so reset it by sending:
You will see the interface leds flash, and then the
IC should respond with ‘ELM327 v1.4b’, followed by a
prompt character. Now, you may choose a protocol
that the ELM327 should connect with, but it is usually
easier to simply select protocol ‘0’ which tells the IC to
search for one:
>AT SP 0
That’s all that you need to do to prepare the
ELM327 for communicating with a vehicle. At the
prompt, issue the mode 01 PID 00 command:
>01 00
The ELM327 should say that it is ‘SEARCHING...’
for a protocol, then it should print a series of numbers,
similar to these:
41 00 BE 1F B8 10
The 41 in the above signifies a response from a
mode 01 request (01 + 40 = 41), while the second
number (00) repeats the PID number requested. A
mode 02, request is answered with a 42, a mode 03
with a 43, etc. The next four bytes (BE, 1F, B8, and
10) represent the requested data, in this case a bit
pattern showing the PIDs that are supported by this
mode (1=supported, 0=not). Although this information
is not very useful for the casual user, it does prove that
the connection is working.
Another example requests the current engine
coolant temperature (ECT). Coolant temperature is
PID 05 of mode 01, and can be requested as follows:
>01 05
The response will be of the form:
41 05 7B
The 41 05 shows that this is a response to a
mode 1 request for PID 05, while the 7B is the desired
data. Converting the hexadecimal 7B to decimal, one
gets 7 x 16 + 11 = 123. This represents the current
temperature in degrees Celsius, but with the zero
offset to allow for subzero temperatures. To convert to
the actual coolant temperature, you need to subtract
40 from the value obtained. In this case, then, the
coolant temperature is 123 - 40 or 83°C.
A final example shows a request for the engine
rpm. This is PID 0C of mode 01, so at the prompt type:
>01 0C
If the engine is running, the response might be:
41 0C 1A F8
The returned value (1A F8) is actually a two byte
hex number that must be converted to a decimal value
to be useful. Converting it, we get a value of 6904,
which seems like a very high value for engine rpm.
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Talking to the Vehicle (continued)
That is because rpm is sent in increments of 1/4 rpm!
To convert to the actual engine speed, we need to
divide the 6904 by 4. A value of 1726 rpm is much
more reasonable.
Note that these examples asked the vehicle for
information without regard for the type of OBD protocol
that the vehicle uses. This is because the ELM327
takes care of all of the data formatting and translation
for you. Unless you are going to do more advanced
functions, there is really no need to know what the
protocol is.
The above examples showed only a single line of
response for each request, but the replies often
consist of several separate messages, either from
multiple ECUs responding, or from one ECU providing
messages that need to be combined to form one
response (see ‘Multiline Responses’ on page 42). In
order to be adaptable to this variable number of
responses, the ELM327 normally waits to see if any
more are coming. If no response arrives within a
certain time, it assumes that the ECU is finished. This
same timer is also used when waiting for the first
response, and if that never arrives, causes ‘NO DATA’
to be printed.
Version 1.3 of the ELM327 introduced a way to
speed up the retrieval of information, if you know how
many responses to expect. By telling the ELM327 how
many lines of data to receive, it knows when it is
finished, so does not have to go through the final
timeout, waiting for data that is not coming. Simply add
a single hex digit after the OBD request bytes - the
value of the digit providing the maximum number of
responses to obtain, and the ELM327 does the rest.
For example, if you know that there is only one
response coming for the engine temperature request
that was previously discussed, you can now send:
>01 05 1
and the ELM327 will return immediately after obtaining
only one response. This may save a considerable
amount of time, as the default time for the AT ST timer
is 200 msec. (The ELM327 still sets the timer after
sending the request, but that is only in case the single
response does not arrive.)
Some protocols (like J1850 PWM) require an
acknowledgement from the ELM327 for every
message sent. If you provide a number for the
responses that is too small, the ELM327 will return to
the prompt too early, and you may cause bus
congestion while the ECU tries several times to resend
the messages that were not acknowledged. For this
reason, you must know how many responses to
expect before using this feature.
As an example, consider a request for the vehicle
identification number (VIN). This number is 17 digits
long, and typically takes 5 lines of data to be
represented. It is obtained with mode 09, PID 02, and
should be requested with:
>09 02
or with:
>09 02 5
if you know that there are five lines of data coming. If
you should mistakenly send 09 02 1, you might cause
This ability to specify the number of responses
was really added with the programmer in mind. An
interface routine can determine how many responses
to expect for a specific request, and then store that
information for use with subsequent requests. That
number can then be added to the requests and the
response time can be optimized. For an individual
trying to obtain a few trouble codes, the savings are
not really worth the trouble, and it’s easiest to use the
old way to make a request.
We offer one additional warning when trying to
optimize the speed at which you obtain information
from vehicles. Prior to the APR2002 release of the
J1979 standard, sending J1850 requests more
frequently than every 100 msec was forbidden. With
the APR2002 update, scan tools were allowed to send
the next request without delay if it was determined that
all the responses to the previous request had been
received. Vehicles made prior to this time may not be
able to tolerate requests at too fast a rate, so use
caution with them.
Hopefully this has shown how typical requests are
made using the ELM327. If you are looking for more
information on modes and PIDs, it is available from
the SAE (www.sae.org), from ISO (www.iso.org), or
from various other sources on the web.
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Interpreting Trouble Codes
Likely the most common use that the ELM327 will
be put to is in obtaining the current Diagnostic Trouble
Codes (or DTCs). Minimally, this requires that a mode
03 request be made, but first one should determine
how many trouble codes are presently stored. This is
done with a mode 01 PID 01 request as follows:
>01 01
To which a typical response might be:
41 01 81 07 65 04
The 41 01 signifies a response to the request, and
the next data byte (81) is the number of current trouble
codes. Clearly there would not be 81 (hex) or 129
(decimal) trouble codes present if the vehicle is at all
operational. In fact, this byte does double duty, with
the most significant bit being used to indicate that the
malfunction indicator lamp (MIL, or ‘Check Engine
Light’) has been turned on by one of this module’s
codes (if there are more than one), while the other 7
bits of this byte provide the actual number of stored
trouble codes. In order to calculate the number of
stored codes when the MIL is on, simply subtract 128
(or 80 hex) from the number.
The above response then indicates that there is
one stored code, and it was the one that set the Check
Engine Lamp or MIL on. The remaining bytes in the
response provide information on the types of tests
supported by that particular module (see the J1979
document for further information).
In this instance, there was only one line to the
response, but if there were codes stored in other
modules, they each could have provided a line of
response. To determine which module is reporting the
trouble code, one would have to turn the headers on
(AT H1) and then look at the third byte of the three
byte header for the address of the module that sent
the information.
Having determined the number of codes stored,
the next step is to request the actual trouble codes
with a mode 03 request (there is no PID needed):
A response to this could be:
43 01 33 00 00 00 00
6 bytes in the response have to be read in pairs to
show the trouble codes (the above would be
interpreted as 0133, 0000, and 0000). Note that the
response has been padded with 00’s as required by
the SAE standard for this mode – the 0000’s do not
represent actual trouble codes.
As was the case when requesting the number of
stored codes, the most significant bits of each trouble
code also contain additional information. It is easiest to
use the following table to interpret the extra bits in the
first digit as follows:
If the first hex digit received is this,
Replace it with these two characters
Powertrain Codes - SAE defined
“ - manufacturer defined
“ - SAE defined
“ - jointly defined
Chassis Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - reserved for future
Body Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - reserved for future
Network Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - reserved for future
Taking the example trouble code (0133), the first
digit (0) would then be replaced with P0, and the 0133
reported would become P0133 (which is the code for
an ‘oxygen sensor circuit slow response’). Note that
the ISO 15765-4 (CAN) protocol is very similar, but it
adds an extra data byte (in the second position),
showing how many data items (DTCs) are to follow.
To provide a few more examples, if the received
code was D016, you would replace the D with U1, and
the resulting trouble code would be U1016. Similarly,
1131 received would actually be for the code P1131.
The ‘43’ in the above response simply indicates
that this is a response to a mode 03 request. The other
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Resetting Trouble Codes
The ELM327 is quite capable of resetting
diagnostic trouble codes, as this only requires issuing
a mode 04 command. The consequences should
always be considered before sending it, however, as
more than the MIL (or ‘Check Engine Light’) will be
reset. In fact, issuing a mode 04 will:
- reset the number of trouble codes
- erase any diagnostic trouble codes
- erase any stored freeze frame data
- erase the DTC that initiated the freeze frame
- erase all oxygen sensor test data
- erase mode 06 and 07 information
- not erase permanent (mode 0A) trouble codes
(these are reset by the ECU only)
Clearing of all of this data is not unique to the
ELM327 – it occurs whenever any scan tool is used to
reset the codes. The biggest problem with losing this
data is that your vehicle may run poorly for a short
time, while it performs a recalibration.
To avoid inadvertently erasing stored information,
the SAE specifies that scan tools must verify that a
mode 04 is intended (‘Are you sure?’) before actually
sending it to the vehicle, as all trouble code
information is immediately lost when the mode is sent.
Remember that the ELM327 does not monitor the
content of the messages, so it will not know to ask for
confirmation of the mode request – this would have to
be the duty of a software interface, if one is written.
As stated, to actually erase diagnostic trouble
codes, one need only issue a mode 04 command. A
response of 44 from the vehicle indicates that the
mode request has been carried out, the information
erased, and the MIL turned off. Some vehicles may
require a special condition to occur (eg. the ignition on
but the engine must not be running) before they will
respond to a mode 04 command.
That is all there is to clearing trouble codes. Once
again, do not accidentally send the 04 code!
Quick Guide for Reading Trouble Codes
If you do not use your ELM327 for some time, this
entire data sheet may seem like quite a bit to review
when your ‘Check Engine’ light eventually comes on,
and you just want to know why. We offer this section
as a quick guide to the basics that you will need.
To get started, connect the ELM327 circuit to your
PC or PDA and communicate with it using a terminal
program such as HyperTerminal, ZTerm, ptelnet, or a
similar program. It should normally be set to either
9600 or 38400 baud, with 8 data bits, and no parity or
The chart at the right provides a quick procedure
on what to do next:
Ignition Key to ON,
but vehicle not running
to see how many codes
(2nd digit of the 3rd byte)
to see the codes
(ignore the first byte and
read the others in pairs)
to reset the codes
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Bus Initiation
Both the ISO 9141-2 and ISO 14230-4 (KWP2000)
standards require that the vehicle’s OBD bus be
initialized before any communications can take place.
The ISO 9141 standard allows for only a slow (2 to 3
second) initiation process, while ISO 14230 allows for
both a slow method, and a faster alternative.
The ELM327 will perform this bus initiation for you,
but generally not until a request needs to be sent (you
can force one with the FI and SI commands). If the bus
initiation occurs during an automatic search, you will
not see any status reporting, but if you have the Auto
option off (and are set to protocols 3, 4, or 5), then you
will see a message similar to this:
The three dots appear only as the slow initiation
process is carried out – a fast initiation does not show
the dots. This will be followed by either the expression
‘OK’ to say it was successful, or else an error message
to indicate that there was a problem. (The most
common error encountered is in forgetting to turn the
vehicle’s key to ‘ON’ before attempting to talk to the
Once the bus has been initiated, communications
must take place regularly (typically at least once every
five seconds), or the bus will revert to a low-power
‘sleep’ mode. If you are not sending data requests
often enough, the ELM327 will generate requests for
you to ensure that the bus stays ‘awake’. You will
never see the responses to these, but you may see
the transmit LED flash periodically when these are
being sent.
By default, the ELM327 ensures that these
‘wakeup’ or ‘idle’ messages are sent every 3 seconds,
but this is adjustable with the AT SW command. The
contents of the wakeup message are also user
programmable with the AT WM command, if you
should wish to change them. Users generally do not
have to change either of the above though, as the
default settings work well with almost all systems.
Wakeup Messages
After an ISO 9141 or ISO 14230 connection has
been established, there needs to be periodic data
transfers in order to maintain that connection, and
prevent it from ‘going to sleep.’ If normal requests and
responses are being sent, that is usually sufficient, but
the ELM327 occasionally has to create its own
messages to prevent the connection from timing out.
We term these periodic messages that are created
the ‘Wakeup Messages’, as they keep the connection
alive, and prevent the circuitry from going back to an
idle or sleep mode. (Some texts refer to these
messages simply as ‘idle messages.’) The ELM327
automatically creates and sends these for you if there
appears to be no other activity – there is nothing that
you need do to ensure that they occur. To see these,
once a connection is made, simply monitor the OBD
transmit LED – you will see the periodic ‘blips’ created
when the ELM327 sends one. If you are curious as to
the actual contents of the messages, you can then
perform a Buffer Dump to see the bytes. Note that the
ELM327 never obtains or prints a response to any of
the wakeup messages.
The standards state that if there is no activity at
least every five seconds, the data connection may
close. To ensure that this does not happen, and to
provide some margin, the ELM327 will send a wakeup
message after three seconds of no activity. This time
interval is fully programmable, should you prefer a
different setting (see the AT SW command).
As with the ELM323, the ELM327 allows users to
change the actual wakeup message that is sent. To do
so, simply send the ELM327 a Wakeup Message
command, telling it what you wish the message to be
changed to. For example, if you would like to send the
data bytes 44 55 with the header bytes set to 11 22
33, simply send the command:
>AT WM 11 22 33 44 55
and from that point forward, every wakeup message
that the ELM327 sends will be as shown. You should
not provide a checksum byte, as it will be automatically
added for you.
You can change these as often as you want, the
only restriction being that every time you do, you must
provide the complete message – the header bytes and
the data bytes (the current version of the ELM327 only
allows for messages of one to six bytes in length).
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Selecting Protocols
The ELM327 supports several different OBD
protocols (see Figure 2, at right). As a user, you may
never have to choose which one it should use (since
the factory settings cause an automatic search to be
performed for you), but while experimenting, you may
want to specify a protocol to be used.
For example, if you know that your vehicle uses
the SAE J1850 VPW protocol, you may want the
ELM327 to use only that protocol, and no others. If
that is what you want, simply determine the protocol
number (from Figure 2), then use the ‘Set Protocol’ AT
Command as follows:
>AT SP 2
From this point on, the default protocol (used after
every power-up or AT D command) will be protocol 2
(or whichever one that you have chosen). Verify this
by asking the ELM327 to describe the protocol:
Now what happens if your friend has a vehicle that
uses ISO 9141-2? How do you now use the ELM327
interface for that vehicle, if it is set for J1850?
One possibility is to change your protocol selection
to allow for the automatic searching for another
protocol, on failure of the current one. This is done by
putting an ‘A’ before the protocol number, as follows:
SAE J1850 PWM (41.6 kbaud)
SAE J1850 VPW (10.4 kbaud)
ISO 9141-2 (5 baud init)
ISO 14230-4 KWP (5 baud init)
ISO 14230-4 KWP (fast init)
ISO 15765-4 CAN (11 bit ID, 500 kbaud)
ISO 15765-4 CAN (29 bit ID, 500 kbaud)
ISO 15765-4 CAN (11 bit ID, 250 kbaud)
ISO 15765-4 CAN (29 bit ID, 250 kbaud)
SAE J1939 CAN (29 bit ID, 250* kbaud)
User1 CAN (11* bit ID, 125* kbaud)
User2 CAN (11* bit ID, 50* kbaud)
*user adjustable
Figure 2. ELM327 Protocol Numbers
memory will only occur after a valid protocol is found,
and only if the memory function is enabled (M0/M1).
For the previous example, all that needs to be sent is:
Now, the ELM327 will try protocol 2, but will then
automatically begin searching for another protocol
should the attempt to connect with protocol 2 fail (as
would happen when you try to connect to your friend’s
The Set Protocol commands cause an immediate
write to the internal EEPROM, before even attempting
to connect to the vehicle. This write is time-consuming,
affects the setting for the next powerup, and may not
actually be appropriate, if the protocol selected is not
correct for the vehicle. To allow a test before a write
occurs, the ELM327 offers one other command - the
Try Protocol (TP) command.
Try Protocol is very similar to Set Protocol. It is
used in exactly the same way as the AT SP command,
the only difference being that a write to internal
Many times, it is very difficult to even guess at a
protocol to try first. In these cases, it is best to simply
let the ELM327 decide what to use. This is done by
telling it to use protocol 0 (with either the SP or the TP
To have the ELM327 automatically search for a
protocol to use, simply send:
>AT SP 0
and when the next OBD command is to be sent, the
ELM327 will automatically look for one that responds.
You will see a ‘SEARCHING...’ message, followed by
a response, after which you can ask the ELM327 what
protocol it found (by sending AT DP).
The first versions of the ELM327 used the SAE
recommended search order (protocol 1, 2, 3, etc.), but
recent versions of the IC modify the search order
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Selecting Protocols (continued)
based on any active inputs that are present. If you
need to follow the SAE J1978 order, you should send
the ELM327 an AT SS command, or step through
each protocol with the TP command.
The automatic search works well with OBDII
systems, but may not be what you need if you are
experimenting. During a search, the ELM327 ignores
any headers that you have previously defined (since
there is always a chance that your headers may not
result in a response), and it uses the default OBD
header values for each protocol. It will also use
standard requests (ie 01 00) during the searches. If
this is not what you want, the results may be a little
To use your own header (and data) values when
attempting to connect to an ECU, do not tell the
ELM327 to use protocol 0. Instead, tell it to either use
only your target protocol (ie. AT SP n), or else tell it to
use yours with automatic searches allowed on failure
(ie AT SP An). Then send your request, with headers
assigned as required. The ELM327 will then attempt to
connect using your headers and your data, and only if
that fails (and you have chosen the protocol with AT
SP An) will it search using the standard OBD default
In general, 99% of all users find that enabling the
memory (setting pin 5 to 5V) and using the ‘Auto’
option when searching (you may need to send AT
SP 0) works very well. After the initial search, the
protocol used by your vehicle becomes the new
default, but it is still able to search for another, without
your having to say AT SP 0 again.
OBD Message Formats
To this point we have only discussed the contents
(data portion) of an OBD message, and made only
passing mention of other parts such as headers and
checksums, which all messages use to some extent.
On Board Diagnostics systems are designed to be
very flexible, providing a means for several devices to
communicate with one another. In order for messages
to be sent between devices, it is necessary to add
information describing the type of information being
sent, the device that it is being sent to, and perhaps
which device is doing the sending. Additionally, the
importance of the message becomes a concern as
well – crankshaft position information is certainly of
considerably more importance to a running engine
than a request for the number of trouble codes stored,
3 header bytes
or the vehicle serial number. So to convey importance,
messages are also assigned a priority.
The information describing the priority, the
intended recipient, and the transmitter are usually
needed by the recipient even before they know the
type of request that the message contains. To ensure
that this information is obtained first, OBD systems
transmit it at the start (or head) of the message. Since
these bytes are at the head, they are usually referred
to as header bytes. Figure 3 below shows a typical
OBD message structure that is used by the
SAE J1850, ISO 9141-2, and ISO 14230-4 standards.
It uses 3 header bytes as shown, to provide details
concerning the priority, the receiver, and the
transmitter. Note that many texts refer to the receiver
up to 7 data bytes
Figure 3. An OBD Message
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OBD Message Formats (continued)
as the ‘Target Address’ (TA), and the transmitter as
the ‘Source Address’ (SA).
Another concern when sending any message is
that errors might occur in the transmission, and the
received data may be falsely interpreted. To detect
errors, the various protocols all provide some form of
check on the received data. This may be as simple as
a sum calculation (ie a ‘running total’ of byte values)
that is sent at the end of a message. If the receiver
also calculates a sum as bytes are received, then the
two values can be compared and if they do not agree,
the receiver will know that an error has occurred.
Since simple sums might not detect multiple errors, a
more reliable (and more complicated) sum called a
Cyclic Redundancy Check (or ‘CRC’) is often used. All
of the protocols specify how errors are to be detected,
and the various ways of handling them if they occur.
The OBD data bytes are thus normally
encapsulated within a message, with ‘header’ bytes at
the beginning, and a ‘checksum’ at the end. The
J1850, ISO 9141-2, and ISO 14230-4 protocols all use
essentially the same structure, with three header
bytes, a maximum of seven data bytes and one
checksum byte.
The ISO 15765-4 (CAN) protocol uses a very
similar structure (see Figure 4, below), the main
‘header’ bytes
ID bits (11 or 29)
difference really only relating to the structure of the
header. CAN header bytes are not referred to as
header bytes – they are called ‘ID bits’ instead. The
initial CAN standard defined the ID bits as being 11 in
number, while the more recent CAN standard now
allows for either 11 or 29 bit IDs.
The ELM327 does not normally show any of these
extra bytes unless you turn that feature on with the
Headers On command (AT H1). Issuing that allows
you to see the header bytes and the checksum byte
(for the J1850, ISO 9141 and ISO 14230 protocols).
For the CAN protocols, you will see the ID bits, and
other items which are normally hidden such as the PCI
byte for ISO 15765, or the data length codes (if they
are enabled with PP 29, or AT D1). Note that the
ELM327 does not display the checksum information
for CAN systems, or the IFR bytes for J1850 systems.
It is not necessary to ever have to set these
header byes, or to perform a checksum calculation, as
the ELM327 will always do this for you. The header
bytes are adjustable however, should you wish to
experiment with advanced messages such as those
for physical addressing. The next section provides a
discussion on how to do this…
data bytes (8 in total)
7 data bytes
Figure 4. A CAN OBD Message
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Setting the Headers
The emissions related diagnostic trouble codes
that most people are familiar with are described in the
SAE J1979 standard (ISO15031-5). They represent
only a portion of the data that a vehicle may have
available – much more can be obtained if you are able
to direct the requests elsewhere.
Accessing most OBDII diagnostics information
requires that requests be made to what is known as a
a ‘functional address.’ Any processor that supports the
function will respond to the request (and theoretically,
many different processors can respond to a single
functional request). In addition, every processor (or
ECU) will also respond to what is known as their
physical address. It is the physical address that
uniquely identifies each module in a vehicle, and
permits you to direct more specific queries to only one
particular module.
To retrieve information beyond that of the OBDII
requirements then, it will be necessary to direct your
requests to either a different functional address, or to
an ECU’s physical address. This is done by changing
the data bytes in the message header.
As an example of functional addressing, let us
assume that you want to request that the processor
responsible for Engine Coolant provide the current
Fluid Temperature. You do not know its address, so
you consult the SAE J2178 standard and determine
that Engine Coolant is functional address 48. SAE
standard J2178 also tells you that for your J1850 VPW
vehicle, a priority byte of A8 is appropriate. Finally,
knowing that a scan tool is normally address F1, you
have enough information to specify the three header
bytes (A8 48 and F1). To tell the ELM327 to use these
new header bytes, all you need is the Set Header
>AT SH A8 48 F1
The three header bytes assigned in this manner
will stay in effect until changed by the next AT SH
command, a reset, or an AT D.
Having set the header bytes, you now need only
send the secondary ID for fluid temperature (10) at the
prompt. If the display of headers is turned off, the
conversation could look like this:
10 2E
The first byte in the response echoes the request,
as usual, while the data that we requested is the 2E
byte. You may find that some requests, being of a low
priority, may not be answered immediately, possibly
causing a ‘NO DATA’ result. In these cases, you may
want to adjust the timeout value, perhaps first trying
the maximum (ie use AT ST FF). Many vehicles will
simply not support these extra addressing modes.
The other, and more common method of obtaining
information is by physical addressing, in which you
direct your request to a specific device, not to a
functional group. To do this, you again need to
construct a set of header bytes that direct your query
to the physical address of the processor, or ECU. If
you do not know the address, recall that the sender of
information is usually shown in the third byte of the
header. By monitoring your system for a time with the
headers turned on (AT H1), you can quickly learn the
main addresses of the senders. The SAE document
J2178 assigns address ranges to these devices if you
are unsure of which might be most appropriate.
When you know the address that you wish to
‘speak to,’ simply use it for the second byte in the
header (assume an address of 10 for this example).
Combine that with your knowledge of SAE J2178 to
choose a priority/type byte (assume a value of E4 for
this example, as if the vehicle is J1850 PWM). Finally,
you need to identify yourself to the target, so that
responses can be returned to you. As is customary for
diagnostic tools, we’ll use an address of F1. As before,
these three bytes are then assigned to the header with
the set header command:
>AT SH E4 10 F1
From this point on, all messages that the ELM327
sends will use these three bytes for the header. All that
needs to be done now is to request data from the
vehicle. For physical addressing, this is often done
using mode 22:
>22 11 6B
62 11 6B 00 00
The response to this command is of the same
format to those seen for ‘standard’ OBD requests. The
request has been repeated (with 40 added to the
mode value in order to show that this is a response),
and this is followed by the actual data (00 00 in this
case). The PIDs used with mode 22 are usually
proprietary to each manufacturer and are generally not
published widely, so you may have difficulty in
determining the ones to use with your vehicle. Elm
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Setting the Headers (continued)
Electronics does not maintain lists of this information,
and cannot provide any further details for you. Mode
22 and others are described in more detail in the SAE
standards document J2190, ‘Enhanced E/E Diagnostic
Test Modes’.
The ISO14230-4 standard defines its header bytes
a little differently. Advanced experimenters will be
aware that for ISO 14230-4, the first header byte must
always include the length of the data field, which
varies from message to message. From that, one
might assume that the you would need to redefine the
header for every message that is to be sent – not so!
The ELM327 always determines the number of bytes
that you are sending, and inserts that length for you, in
the proper place for the header that you are using. If
you are using the standard ISO 14230-4 header, the
length will be put into the first header byte, and you
need only provide the two (most significant) bits of this
byte when defining the header. What you place in the
rest of the byte will be ignored by the ELM327 unless
you set it to 0. If it is 0, it is assumed that you are
experimenting with KWP four byte headers, and the
ELM327 then creates the fourth header byte for you.
Again, you do not need to provide any length to be put
into this byte – it is done for you.
Addressing within the CAN (ISO 15765-4)
protocols is quite similar in many ways. First, consider
the 29 bit standard. The ELM327 splits the 29 bits into
a CAN Priority byte and the three header bytes that we
are now familiar with. This is how they are combined
for use by the ELM327:
>AT CP vv
>AT SH xx yy zz
>AT SH xx yy zz
11 bit ID
5 bits only
Setting an 11 bit (standard) CAN ID
29 bit ID
Setting a 29 bit (extended) CAN ID
The CAN standard states that for diagnostics, the
priority byte (‘vv’ in the diagram) will always be 18 (it is
the default value used by the ELM327). Since it is
rarely changed, it is assigned separately from the
other header bytes, using the CP command.
The next byte (‘xx’) describes the type of message
that this is, and is set to hex DB for functional
addressing, and to DA if using physical addressing.
The next two bytes are as defined previously for the
other standards – ‘yy’ is the receiver (or Target
Address), and ‘zz’ is the transmitter (or Source
Address). For the functional diagnostic requests, the
receiver is always 33, and the transmitter is F1, which
is very similar to ISO 14230-4.
Those that are familiar with the SAE J1939
standard will likely find this header structure to be very
similar (J1939 is a CAN standard for use by ‘heavyduty vehicles’ such as trucks and buses). It uses
slightly different terminology, but there is a direct
parallel between the bytes used by J1939 for the
headers and the grouping of the bytes in the ELM327.
Pages 48 and 49 provide more details of the J1939
message structure.
The final header format to discuss is that used in
11 bit CAN systems. They also use a priority/address
structure, but shorten it into roughly three nibbles
rather than three bytes. The ELM327 uses the same
commands to set these values as for other headers,
except that it only uses the 11 least significant (‘rightmost’) bits of the provided header bytes, and ignores
the others, as shown here:
It quickly becomes inconvenient to have to enter
six digits when only three are required, so there is a
special ‘short’ version of the AT SH command that
uses only three hex digits. It actually operates by
simply adding the leading zeros for you.
The 11 bit CAN standard typically makes
functional requests (ID/header = 7DF), but receives
physical replies (7En). With headers turned on, it is a
simple matter to learn the address of the module that
is replying, then use that information to make physical
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Setting the Headers (continued)
requests if desired. For example, if the headers are on,
and you send 01 00, you might see:
>01 00
7E8 06 41 00 BE 3F B8 13 00
The 7E8 shows that ECU#1 was the one
responding. In order to talk directly to that ECU, all you
need do is to set the header to the appropriate value (it
is 7E0 to talk to the 7E8 device – see ISO 15765-4 for
more information). From that point on, you can ‘talk’
directly to the ECU using its physical address, as
shown here:
>AT SH 7E0
>01 00
7E8 06 41 00 BE 3F B8 13 00
>01 05
7E8 03 41 05 46 00 00 00 00
Hopefully this has helped to get you started. As we
often tell those that write for help – if you are planning
to do some serious experimenting with OBD, you
should buy the relevant standards.
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Monitoring the Bus
Some vehicles use the OBD bus for information
transfer during normal vehicle operation, passing a
great deal of information over it. A lot can be learned if
you have the good fortune to connect to one of these
vehicles, and are able to decipher the contents of the
To see how your vehicle uses the OBD bus, you
can enter the ELM327’s ‘Monitor All’ mode, by sending
the command AT MA from your terminal program. This
will cause the IC to display any information that it sees
on the OBD bus, regardless of transmitter or receiver
addresses (it will show all). Note that the ELM327
remains silent while monitoring, so periodic ‘wakeup’
messages are not sent (if you have an ISO 9141 or
ISO 14230 bus that had been initialized previously, it
may ‘go to sleep’), IFRs are not sent, and the CAN
module does not acknowledge messages.
The monitoring mode can be stopped by putting a
logic low level on the RTS pin, or by sending a single
RS232 character to the ELM327. Any convenient
character can be used to interrupt the IC – there are
no restrictions on whether it is printable, etc. Note that
any character that you send will be discarded, and will
have no effect on any subsequent commands.
The time it takes to respond to such an interrupt
will depend on what the ELM327 is doing when the
character is received. The IC will always finish a task
that is in progress (printing a line, for example) before
printing ‘STOPPED’ and returning to wait for your
input, so it is best to wait for the prompt character (‘>’)
to be sent, or the Busy line to go low, before beginning
to send a new command.
One unexpected result may occur if you have the
automatic protocol search feature enabled, and you
tell the ELM327 to begin monitoring. If the bus is quiet,
the ELM327 will begin searching for an active protocol,
which may not be what you were expecting. Be aware
also that the ISO 9141 and ISO 14230 protocols look
identical when monitoring, so the ELM327 may stop
searching at ISO 9141, even if the actual protocol is
ISO 14230. With the Automatic searching enabled, this
should correct itself, however, when an OBD request
is later made.
If the ‘Monitor All’ command provides too much
information (it certainly does for most CAN systems),
then you can restrict the range of data that is to be
displayed. Perhaps you only want to see messages
that are being transmitted by the ECU with address 10.
To do that, simply type:
and all messages that contain 10 in the third byte of
the header will be displayed.
Using this command with 11 bit CAN systems can
be a little confusing at first. Recall the way in which all
header bytes are stored within the ELM327. An 11 bit
CAN ID is actually stored as the least significant 11
bits in the 3 byte ‘header storage’ location. It will be
stored with 3 bits in the receiver’s address location,
and the remaining 8 bits in the transmitter’s address
location. For this example, we have requested that all
messages created by transmitter ‘10’ be printed, so all
11 bit CAN IDs that end in 10 will be displayed (ie all
that look like ‘x10’).
The other monitoring command that is very useful
is the AT MR command, which looks for specific
addresses in the middle byte of the header. Using this
command, you can look for all messages being sent to
a particular address. For example, to use it to look for
messages being sent to the ECU with address 10,
simply send:
>AT MR 10
and all messages that contain 10 in the second byte of
the header will be displayed.
Using this command with the 11 bit CAN systems
will again need further explanation. It may be helpful to
first picture the hex number ‘10’ in the above example
as the binary number ‘0001 0000’. Recall from above
that 11 bit CAN IDs are actually stored as the least
significant 11 bits in the 3 byte ‘header storage’
locations, and only 3 bits are actually stored in the
middle byte (receiver’s address) position. When
comparing the received CAN ID to the address you
provide with the MR command then, only the rightmost 3 bits of your MR address are considered and
the other 5 bits are ignored. In this example, the
AT MR 10 effectively becomes AT MR 0 for 11 bit
CAN systems, and so all messages that begin with ‘0’
as the first digit will be displayed.
In order to use the AT MR command with CAN 11
bit identifiers, you should always try to use the format
‘AT MR 0x’, where ‘x’ is the digit that you want the
identifiers to begin with. To look for all 2xx’s, use the
command ‘AT MR 02’, and to see all of the 7xx’s, you
should use ‘AT MR 07’.
The ELM327 can be set to automatically send the
Monitor All command to itself after power on, if PP 00
is set to the value 0, and enabled.
>AT MT 10
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CAN Messages and Filtering
Just prior to sending an OBD request, the ELM327
will make sure that the receive filters are set to only
accept certain types of messages, and to reject others.
With the older protocols (J1850, ISO 9141, etc.) the
filters only need to monitor a single byte (much like the
AT MR and MT commands), but with CAN protocols, a
decision typically requires looking at all of the ID bits.
The decision whether to receive a CAN message
or to ignore it is made by what is called an ‘acceptance
filter’. This filter works in conjunction with a ‘mask’,
which is actually a filter on the filter that says which
bits are relevant, and which ones can be ignored. A
few examples might be best to describe how to set the
filter and the mask.
By far the easiest way to set both the filter and the
mask is with the AT CRA command. You don’t really
need to know anything about filters and masks to use
it - simply send the command with the address that
you want to see, and the ELM327 does all the work for
you. For example, if the only messages that you wish
to see are those that have the CAN ID 7E9, then
simply send:
and the ELM327 will set the necessary values so that
all that is displayed are messages with ID 7E9.
If you would like to see a range of values, rather
than a specific ID, then you will need to manually
define the filter and the mask. For example, consider
an application where you are trying to monitor for 29
bit CAN diagnostic messages, exactly like the ELM327
does. By definition, these messages will be sent to the
scan tool at address F1, so from ISO 15765-4, you
know that the ID portion of the reply is of the form:
18 DA F1 xx
where xx is the address of the ECU that is sending the
message. Clearly a filter is needed that requires the
first 21 bits to match, but does not care what the last
eight bits contain.
To create such a filter, we first use the CAN Filter
command to enter the ID values that correspond to the
values that you want to match, and put any value in for
the unknown portion (you will see why in a moment).
For this example, the command would be:
>AT CF 18 DA F1 00
Now, how do you tell the ELM327 to ignore those
last two 0’s? You do that with the mask. The mask is a
bit pattern that tells the ELM327 which bits in the filter
are relevant. If the mask bit is 1, that filter bit is
relevant, and is required to match. If it is 0, then that
filter bit will be ignored. All bits in this filter are relevant,
except those of the last byte. To set the mask then,
you would need to use the CAN Mask command, and
put 1’s for all but the last byte:
>AT CM 1F FF FF 00
After setting the mask and filter in this way, only
IDs that start with 18 DA F1 will be accepted by the
ELM327, and all others will be ignored. Be careful
when experimenting with this, as you will override the
default settings, and you might stop ‘seeing’ any
replies to the requests.
The 11 bit CAN IDs are treated in much the same
manner. Recall that they are stored internally in the
right-most 11 bits of the locations used for 29 bit CAN,
which must be considered when creating a filter or
mask. As an example, assume that we wish to display
all messages that have a 6 as the first digit of the 11
bit ID. We need to set a filter to look for 6 in that digit:
>AT CF 00 00 06 00
The 11 bit ID is stored in the last three locations,
so the 6 would appear where it is shown. Now, to
make only that digit relevant, we create the mask:
>AT CM 00 00 0F 00
Technically, this one digit actually represents 3
bits of the 11 bit ID, so we should not use F (ie 1111)
for the mask, but the ELM327 will only look at the 11
bits, and does not care what we put in the 12th bit (so
we can be lazy).
Clearly, trying to insert 11 bits into the right spot in
a 29 bit ID can be quite cumbersome. To help with
that, the ELM327 offers some 11 bit versions of the CF
and CM commands, which can be used as follows for
the above commands:
>AT CF 600
>AT CM F00
Again, only the 11 least significant (right-most)
digits are actually used, and the ELM327 ignores the
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CAN Messages and Filtering (continued)
12th bit, so you do not need to take special care with
With a little practice, these commands are fairly
easy to master. Initially, try entering the filter and mask
values, then use a command such as AT MA to see
what the results are. The ELM327 knows that you are
trying to filter, and combines the effects of both
commands (it will do that for MR and MT as well). The
MA, MR and MT commands all have the extra benefit
that while they are in effect, the ELM327 will remain
quiet, not sending acknowledgement or error signals,
so anything you do while monitoring should not disrupt
other devices that are on the bus.
Note that if a filter has been set, it will be used for
all CAN messages, so setting filters and masks may
cause standard OBD requests to be ignored, and you
may begin seeing ‘NO DATA’ replies. If this happens,
and you are unsure of why, you may want to reset
everything to the default values (with AT D or possibly
AT WS) and start over.
Multiline Responses
There are occasions when a vehicle must respond
with more information than one ‘message’ is able to
show. In these cases, it responds with several lines
which the receiver must assemble into one complete
One example of this is a request for the 17 digit
vehicle identification number, or VIN. This is available
from newer vehicles using a mode 09, PID 02 request
(but was not initially a requirement, so many older
vehicles do not support it). Here is one example of a
response that might be obtained from a J1850 vehicle:
49 02
49 02
49 02
49 02
49 02
The first two bytes (49 and 02) on each line of the
response are used to show that the information is in
reply to an 09 02 request. The next byte shows which
response it is, while the remaining four bytes are the
data bytes that are being sent. Assembling the data in
the order specified by the third byte, and ignoring the
first few 00’s (they are filler bytes - see J1979) gives:
31 44 34 47 50 30 30 52 35 35 42 31
32 33 34 35 36
The data values actually represent the ASCII
codes for the digits of the VIN. Using an ASCII table to
convert these into characters gives the following VIN
for the vehicle:
1 D 4 G P 0 0 R 5 5 B 1 2 3 4 5 6
CAN systems will display this information in a
somewhat different fashion. Here is a typical response
from a CAN vehicle:
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
The CAN Formatting has been left on (the default),
making the reading of the data easier. With formatting
on, the sequence numbers are shown with a colon (‘:’)
after each. CAN systems add this single hex digit (it
goes from 0 to F then repeats), to aid in reassembling
the data, instead of the byte value that the J1850
vehicle did.
The first line of this response says that there are
014 bytes of information in total. That is 14 in hex, or
20 in decimal terms, which agrees with the 6 + 7 + 7
bytes shown on the three lines. The VIN numbers are
generally 17 digits long, however, so how do we
assemble the number from 20 digits?
This is done by discarding the first three bytes of
the message. The first two are the familiar 49 02, as
this is a response to an 09 02 request, so are not part
of the VIN. The third byte (the ‘01’), tells the number of
data items that are to follow (the vehicle can only have
one VIN, and this agrees with that). Ignoring the third
byte leaves 17 data bytes which are the serial number
(purposely chosen to be identical to the those of the
previous example). All that is needed is a conversion
to ASCII, in order to read them, exactly as before.
From these two examples, you can see that the
format of the data received may depend on the
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Multiline Responses (continued)
protocol used to transmit it. For this reason, a copy of
the SAE J1979 standard would be essential if you are
planning to do a lot of work with this, for example if you
were writing software to display the received data.
The following shows an example of a different type
of multiline response that can occur when two or more
ECUs respond to one request. Here is a typical
response to an 01 00 request:
turn on the headers, and repeat your request:
>AT H1
>01 00
41 00 BE 3E B8 11
41 00 80 10 80 00
This is difficult to decipher without knowing a little
more information. First, turn the headers on to actually
see ‘who’ is doing the talking:
>AT H1
>01 00
48 6B 10 41 00 BE 3E B8 11 FA
48 6B 18 41 00 80 10 80 00 C0
Now, if you analyze the header, you can see that
the third byte shows ECU 10 (the engine controller)
and ECU 18 (the transmission) are both responding
with a reply to the 01 00 request. With modern
vehicles, this type of response occurs often, and you
should be prepared for it.
A final example shows how similar messages
might occasionally be ‘mixed up’ in a CAN system. We
ask the vehicle for the Calibration ID (09 04) and are
presented with the following response:
>09 04
0: 49 04
1: 32 38
0: 49 04
2: 00 00
1: 32 38
2: 00 00
01 35 36 30
39 34 39 41 43
00 31
41 43
00 00
which is quite confusing. The first group (the 013, 0:, 1:
group) seems to make some sense (but the number of
data bytes do not agree with the response), and the
following group is very confusing, as it has two
segment twos. It seems that two ECUs are responding
and the information is getting mixed up. Which ECU do
the responses belong to? The only way to know is to
This time, the order appears to be the same, but
be aware that it may not be – that is why the standard
requires that sequence codes be transmitted with
multiline responses.
Looking at the first digits of these responses, you
can see that some begin with 7E8, and some begin
with 7E9, which are the CAN IDs representing ECU#1
and ECU#2, respectively. Grouping the responses by
ECU gives:
7E8 10 13 49 04 01 35 36 30
7E8 21 32 38 39 34 39 41 43
7E8 22 00 00 00 00 00 00 31
7E9 10 13 49 04 01 35 36 30
7E9 21 32 38 39 35 34 41 43
7E9 22 00 00 00 00 00 00 00
From these, the messages can be assembled in
their proper order. To do this, look at the byte following
the CAN ID - it is what is known as the PCI byte, and
is used to tell what type of data follows. In this case,
the PCI byte begins with either a 1 (for a ‘First Frame’
message), or a 2 (for the ‘Consecutive Frames’). The
second half of the PCI byte shows the order in which
the information is to be assembled (ie. the segment
number). In this case, the segment numbers are
already in order, but if they had not been, it would
have been necessary to rearrange the messages to
place them in order.
Each OBD standard has some minor peculiarities,
but hopefully this has helped you with some of the
more common ones. If you are still having trouble, we
urge you to purchase the relevant standard, and study
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CAN Message Formats
The ISO 15765-4 (CAN) standard defines several
message types that are to be used with diagnostic
systems. Currently, there are four that may be used:
the Single Frame
the First Frame (of a multiframe message)
the Consecutive Frame ( ‘ ‘ )
the Flow Control frame
The Single Frame message contains storage for
up to seven data bytes in addition to what is known as
a PCI (Protocol Control Information) byte. The PCI
byte is always the first of the data bytes, and tells how
many data bytes are to follow. If the CAN Auto
Formatting option is on (CAF1) then the ELM327 will
create this byte for you when sending, and remove it
for you when receiving. (If the headers are enabled,
you will see it in the responses.)
If you turn the Auto Formatting off (with CAF0), it
is expected that you will provide all of the data bytes to
be sent. For diagnostics systems, this means the PCI
byte and the data bytes. The ELM327 will not modify
your data in any way, except to add extra padding
bytes for you, to ensure that you always send as many
data data bytes as are required (eight for ISO15765).
You do not need to set the Allow Long (AT AL) option
in order to send eight bytes, as the IC overrides it for
A First Frame message is used to say that a multiframe message is about to be sent, and tells the
receiver just how many data bytes to expect. The
length descriptor is limited to 12 bits, so a maximum of
4095 byes can be received at once using this method.
Consecutive Frame messages are sent after the
First Frame message to provide the remainder of the
data. Each Consecutive Frame message includes a
single hex digit ‘sequence number’ that is used to
determine the order when reassembling the data. It is
expected that if a message were corrupted and resent,
it could be out of order by a few packets, but not by
more than 16, so the single digit is normally more than
adequate. As seen previously, the serial number for a
vehicle is often a multiframe response:
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
First Frame message. The length (014) was actually
extracted from that message by the ELM327 and
printed on the first line as shown. Following the First
Frame line are two Consecutive Frames (that begin
with 1: and 2:). To learn more details of the exact
formatting, you may want to send a request such as
the one above, then repeat the same request with the
headers enabled (AT H1). This will show the PCI bytes
that are actually used to send these components of the
total message.
The Flow Control frame is one that you do not
normally have to deal with. When a First Frame
message is sent as part of a reply, the ELM327 must
tell the sender some technical things (such as how
long to delay between Consecutive Frames, etc.) and
does so by replying immediately with a Flow Control
message. These are predefined by the ISO 15765-4
standard, so can be automatically inserted for you. If
you wish to generate custom Flow Control messages,
then refer to the ‘Altering Flow Control Messages’
section, on page 46.
If a Flow Control frame is detected while
monitoring, the line will be displayed with ‘FC: ’ before
the data, to help you with decoding of the information.
There is a final type of message that is
occasionally reported, but is not supported by the
diagnostics standard. The (Bosch) CAN standard
allows for the transmission of a data request without
sending any data in the requesting message. To
ensure that the message is seen as such, the sender
also sets a special flag in the message (the RTR bit),
which is seen at each receiver. The ELM327 always
looks for this flag, or for zero data bytes, and may
report to you that an RTR was detected while
monitoring. This is shown by the characters RTR
where data would normally appear, but only if the CAN
Auto Formatting is off, or headers are enabled. Often,
when monitoring a CAN system with an incorrect baud
rate chosen, RTRs may be seen.
Note that the CAN system is quite robust with
several error detecting methods in place, so that
during normal data transmission you will rarely see
any errors. When monitoring buses however, you may
well see errors (especially if the ELM327 is set to an
incorrect baud rate). As a diagnostic aid, when errors
do occur, the ELM327 will print all bytes (no matter
what CAF, etc., is set to), followed by the message
In this example, the line that begins with 0: is the
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Restoring Order
There may be times when it seems the ELM327 is
out of control, and you will need to know how to
restore order. Before we continue to discuss modifying
too many parameters, this seems to be a good point to
discuss how to ‘get back to the start’. Perhaps you
have told the ELM327 to monitor all data, and there
are screens and screens of data flying by. Perhaps the
IC is now responding with ‘NO DATA’ when it did work
previously. This is when a few tips may help.
The ELM327 can always be interrupted from a
task by a single keystroke from the keyboard. As part
of its normal operation, checks are made for received
characters and if found, the IC will stop what it is doing
at the next opportunity. Often this means that it will
continue to send the information on the current line,
then stop, print a prompt character, and wait for your
input. The stopping may not always seem immediate if
the RS232 send buffer is almost full, though – you will
not actually see the prompt character until the buffer
has emptied, and your terminal program has finished
printing what it has received.
There are times when the problems seem more
serious and you don’t remember just what you did to
make them so bad. Perhaps you have ‘adjusted’ some
of the timers, then experimented with the CAN filter, or
perhaps tried to see what happens if the header bytes
are changed. All of these can be reset by sending the
‘set to Defaults’ AT Command:
the four status LEDs in sequence. A much quicker
option is available with the ELM327, however, if the
led test is not required – the ‘Warm Start’ command:
The AT WS command performs a software reset,
restoring exactly the same items as the AT Z does, but
it omits the LED test, making it considerably faster.
Also, it does not affect any baud rates that have been
set with the AT BRD command (which AT Z does), so
is essential if you are modifying the RS232 baud rates
with software.
Any of the above methods should be effective in
restoring order while experimenting. There is always
the chance that you may have changed a
Programmable Parameter, however, and are still
having problems with your system. In this case, you
may want to simply turn off all of the Programmable
Parameters (which forces them to their default values).
To do so, send the command:
which should disable all of the changes that you have
made. Since some of the Programmable Parameters
are only read during a system reset, you may have to
follow this command with a system reset:
This will often be sufficient to restore order, but it
can occasionally bring unexpected results. One such
surprise will occur if you are connected to a vehicle
using one protocol, but the saved (default) protocol is
a different one. In this case, the ELM327 will close the
current session and then change the protocol to the
default one, exactly as instructed.
If the AT D does not bring the expected results, it
may be necessary to do something more drastic - like
resetting the entire IC. There are a few ways that this
can be performed with the ELM327. One way is to
simply remove the power and then reapply it. Another
way that acts exactly the same way as a power off and
then on is to send the full reset command:
after which, you can start over with what is essentially
a device with ‘factory settings’. There may be times
when even this command is not recognized, however.
If that is the case, you will need to use the hardware
method of turning the PPs off. See the section on
‘Programmable Parameters’ (pages 54 and 55) for
more details.
It takes approximately one second for the IC to
perform this reset, initialize everything and then test
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Altering Flow Control Messages
ISO 15765-4 (CAN) provides for only eight data
bytes per frame of data. Of course, there are many
cases where the data which needs to be sent is longer
than 8 bytes, and CAN has made provision for this by
allowing data to be separated into segments, then
recombined at the receiver.
To send one of these multi-line messages, the
transmitter in a CAN system will send a ‘First Frame’
message, and then wait for a reply from the receiver.
This reply, called a ‘Flow Control’ message contains
information concerning acceptable message timing,
etc., and is required to be sent before the transmitter
will send any more data. For ISO 15765-4, the type of
response is well defined, and never changes. The
ELM327 will automatically send this ISO 15765-4 Flow
Control response for you as long as the CAN Flow
Control option is enabled (CFC1), which it is by
Several users have requested that we provide
more flexibility over the data sent in the Flow Control
message, and as of v1.1, we have provided a means
to do this. In order to change how the ELM327
responds when it needs to send a Flow Control
message, you need to change Flow Control ‘modes’.
The default Flow Control mode is number ‘0’. At
any time while you are experimenting, if you should
wish to restore the automatic Flow Control responses
(for ISO 15765-4), simply set the mode to 0:
This will immediately restore the responses to their
default settings.
Mode 1 has been provided for those that need
complete control over their Flow Control messages. To
use it, simply define the CAN ID (header) and data
bytes that you require be sent in response to a First
Frame message. Note that if you try to set the mode
before defining these values, you will get an error:
and then you can set the mode:
From this point on, every First Frame message
received will be responded to with the custom
message that you have defined (7E8 00 11 22 in this
The third mode currently supported allows the
user to set the data bytes which are to be sent, but not
the ID bits. The ID bits (header bytes) in mode 2 are
set to those which were received in the First Frame
message, without change. To use this mode, first
define your data bytes, then activate the mode:
>AT FC SD 00 11 22
For most people, there will be little need to
manipulate these ‘Flow Control’ messages, as the
defaults are designed to work with the CAN OBD
standards. If you wish to experiment, these special AT
commands offer that control for you.
The following chart summarizes the currently
supported flow control modes:
ID Bits &
Data Bytes
no values
no values
ID Bits &
Data Bytes
ID Bits
Data Bytes
Flow Control Modes
You must set the headers and data first:
>AT FC SD 00 11 22
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Using CAN Extended Addresses
Some vehicles with CAN interfaces use a data
format that is slightly different from what we have
described so far. The data packets look very similar,
except that the first byte is used for the receiver’s (ie
target’s) address. The remaining seven bytes are used
as described previously.
We refer to this type of addressing as ‘CAN
Extended Addressing’, and provide support for it with
the AT CEA commands. Perhaps an example would
better describe how to use them.
Here is a portion of a data transfer that was taken
from a vehicle. For the moment, ignore the first data
bytes on each line and only look at the remaining data
bytes (that are outlined in grey):
>AT SH 7B0
Notice that there was a flow control message that
was sent in this group, but it’s not quite the same as
the one for OBD systems. For this reason, you’ll need
to define your own flow control with the following three
statements (we won’t show the OK’s any more, to
save space):
>AT FC SD 04 30 FF 00
The final setup statement that you will need is to
tell the ELM327 to send to CAN Extended Address 04:
>AT CEA 04
Now everything is configured. Next, tell the IC to
use this protocol, and to bypass any initiation (as it is
not standard OBD, and would likely fail):
If you are familiar with the ISO 15765 data format,
you will be able to recognize that the data bytes shown
inside the box seem to conform to the standard. The
rows that begin with 02 are Single Frames, the one
that starts with 10 is a First Frame, while the one with
30 is a Flow Control, and the others are Consecutive
The remaining bytes, shown outside the box, are
the standard 11 bit CAN ID, and an extra address
byte. The lines with F1 for the extra address are
directed to the scan tool (all scan tools generally use
F1 as the default address), and the other lines are
being sent to the vehicle’s module (at address 04).
Version 1.4 of the ELM327 is able to handle these
types of messages, but does require some setup. For
example, if the messages use 11 bit IDs with
ISO 15765 formatting, and the baud rate is 50 kbps,
then the PB command to configure protocol B is:
>AT PB 81 0A
Next, we’ll want to receive all messages with an ID
of 7C0, and send with an ID (header) of 7B0:
That’s all. To exactly reproduce the flow of data
shown, you only need to send the relevant data bytes
and the ELM327 will add the rest:
>10 81
50 81
>21 A2
0: 61 A2
0: DF 01
1: 02 05
2: 09 01
09 01
00 04
01 00
Notice that for some reason, this vehicle has sent
two segment 0’s, but that just means that it doesn’t
exactly follow the ISO 15765 protocol. The above
shows what the responses would look like with
formatting on, and headers off. If you change either,
the data exchange would look more like what we
initially showed.
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SAE J1939 Messages
The SAE J1939 CAN standard is being used by
many types of heavy machinery – trucks, buses, and
agricultural equipment, to name a few. It uses the
familiar CAN (ISO 11898) physical interface, and
defines its own format for data transfer (which is very
similar to the ISO 15765 standard that is used for
The following will discuss a little of how data is
transferred using the J1939 standard. Considerably
more information is provided in the Society of
Automotive Engineers (SAE) standards documents, so
if you are going to be doing a lot of work with J1939, it
may be wise to purchase copies of them. At minimum,
the J1939-73 diagnostics, the J1939-21 data transfer,
and the J1939-71 vehicle application documents
should be purchased. Another great reference for this
work is the HS-J1939 two book set, also available from
the SAE.
The current version of the J1939 standard allows
only one data rate (250 kbps), but work is underway to
amend the standard so that an alternate rate of
500 kbps will also be allowed. For the purpose of this
discussion, the data rate is not important - it is the
format of the information that we will discuss.
All CAN messages are sent in ‘frames’, which are
data structures that have ID bits and data bytes, as
well as checksums and other items. Many of the J1939
frames are sent with eight data bytes, although there is
no requirement to do so (unlike ISO 15765, which
must always send eight data bytes in each frame). If a
J1939 message is eight bytes or less, it will be sent in
one frame, while longer messages are sent using
multiple frames, just like ISO 15765. When sending
multiple frames, a single data byte is used to assign a
‘sequence number’, which helps in determining if a
frame is missing, as well as in the reassembly of the
received message. Sequence numbers always start
with 01, so there is a maximum of 255 frames in a
message, or 1785 bytes.
One major feature of the J1939 standard is its very
orderly, well defined data structures. Related data is
defined and specified in what are called ‘parameter
groups’. Each parameter group is assigned a
‘parameter group number’, or PGN, that uniquely
defines that packet of information. Often, the
parameter groups consist of eight bytes of data (which
is convenient for CAN messages), but they are not
restricted to this. Many of the PGNs, and the data
within them (the SPNs) are defined in the J1939-71
document, and manufacturers also have the ability to
define their own proprietary PGNs.
The ID portion of a J1939 CAN frame is always 29
bits in length. It provides information as to the type of
message that is being sent, the priority of the
message, the device address that is sending it, and
the intended recipient. Information within the ID bits is
divided roughly into byte size pieces as follows:
3 bits 2 bits
8 bits
8 bits
8 bits
PDU1 Format
The data structure formed by the 29 bit ID, and the
associated data bytes is called a Protocol Data Unit, or
PDU. When the ID bits have a destination address
specified, as is shown above, it is said to be a PDU1
Format message.
The two bits shown between the Priority and the
PDU Format are known as the Extended Data Page
(EDP), and the Data Page (DP) bits. For J1939, EDP
must always be set to ‘0’, while the DP bit is used to
extend the range of values that the PDU Format may
have. While the DP bit is typically ‘0’ now, that may not
be true in the future.
Not all J1939 information is sent to a specific
address. In fact, one of the unique features of this
standard is that there is a large amount of information
that is being continually broadcast over the network,
with receivers using it as they see fit. In this way,
multiple devices requiring the same information do not
have to make multiple requests to obtain it, information
is provided at regular time intervals, and bus loading is
If information is being broadcast over the network
to no particular address, then the Destination Address
field is not required. The eight bits can be put to better
use, possibly by extending the PDU Format field. This
is what is done for a PDU2 Format frame, as shown
3 bits 2 bits
8 bits
8 bits
8 bits
PDU2 Format
So how does one know if they are looking at a
PDU1 Format frame that contains an address, or a
PDU2 Format frame that does not? The secret lies in
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the values assigned to the PDU Format field. If the
PDU Format value begins with ‘F’ (when expressed as
a hexadecimal number), it is PDU2. Any other value
for the first digit means that it is a PDU1 Format frame,
which contains an address.
To summarize, PDU1 format frames are sent to a
specific address, and PDU2 frames are sent to all
addresses. To further complicate matters, however,
PDU1 frames may be sent to all addresses. This is
done by sending the message to a special ‘global
address’ which has the value FF. That is, if you see a
PDU1 message (where the first digit of the PDU
Format byte is not an F), and the Destination Address
is FF, then that message is being sent to all devices.
The J1939 recommended practices document
provides a list of addresses that should be used by
devices. It is particularly important to adhere to this list
with the ELM327, as the IC uses a fixed address
method and is not able to negotiate a different one, per
J1939-81. OBD Service Tools should use either F9 or
FA as their address (the ELM327 uses F9). If you wish
to change this, you can use the AT TA (tester address)
command, or simply define it with the header.
The J1939 protocol uses the AT CP and AT SH
commands to assign values for the ID bits, just as the
other CAN protocols do. How these are used was
>AT CP vv
discussed previously, but we will repeat it here, to be
complete. Since the priority (and DP and EDP) values
only rarely change, they are assigned with the CP
command. By default, the ELM327 uses a priority of 6
(binary 110), and sets the EDP and DP to 0. The
default value for the CP setting is then 110+0+0 (which
would be entered as 11000 or 18 in hex). The values
assigned using the SH command relate directly to the
bytes in the J1939 ID, as shown in Figure 5 below.
This has tried to cover the basics of the J1939
message structure, but if you want more information,
you should look at the standards mentioned
previously. One other one that gives good examples of
actual data is J1939-84 which describes the
compliance tests and shows the expected responses.
Even at 250 kbps, J1939 data is transferred at a
rate that is more than ten times faster than the
previous heavy duty vehicle standard (SAE J1708),
and several of the light duty standards. As designers
build more into each system, the amount of
information required continues to grow, however, so
the 500 kbps version of J1939 will be a welcome
>AT SH xx yy zz
5 bits
Figure 5. Setting the J1939 CAN ID
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Using J1939
This section provides a few examples which show
how to monitor an SAE J1939 data bus, and how to
make requests of devices that are connected to it.
To begin, you will need to configure the ELM327
for J1939 operation, at the correct baud rate. Protocol
A is predefined for J1939 at 250 kbps, which is what
most applications require. To use protocol A, send:
Protocols B and C may also be used with J1939,
if you wish to experiment with other baud rates. To use
them for J1939, the option value (in PP 2C or 2E) must
be set to 42, and the baud rate divisor (in PP 2D or 2F)
must be set to the appropriate value. Perhaps the
simplest way to provide an alternate rate is to use the
AT PB command, that allows you to set both the
options byte (which is always 42), and the baud rate
divisor (which is 500k ÷ the desired baud rate) at the
same time. For example, to set protocol B for J1939
operation at 500 kbps, simply send:
>AT PB 42 01
then send:
to begin. Note that this setting will not be maintained if
the IC is reset, so if you want a more permanent
setting, you should store the values in PP 2C and 2D.
Once the protocol is set, then make sure that you
have a long timeout value chosen for the data
receives. If you have a version 1.4b IC, you do not
need to do anything, but if you have a previous
version, we recommend that you send:
wait while the initial response completes (and this
could take more than the normal ST time since
broadcast responses must be spaced at least 50 msec
apart). If you know that a reply should be coming, and
you are seeing ‘NO DATA’ responses, then send
AT JTM5 and try it again, as that may be the problem.
Restore the timer multiplier to normal with AT JTM1.
Once the J1939 protocol is selected, and the
timeout value has been adjusted, the ELM327 is ready
for a command. The first one that we will send is called
a DM1 or ‘diagnostic message 1’, which provides the
currently active diagnostic trouble codes. DM1 is one
of more than 50 predefined diagnostic messages, and
is special in that it is the only one that is broadcast
continually over the bus at regular intervals. The
ELM327 has an AT command that is used to obtain
the DM1 trouble codes:
If you are connected to a vehicle, you should now
see messages printed at one second intervals. If you
are only connected to a single device (for example,
with a simulator on the bench, or to a device with a
single CAN data port), you may see data with
<RX ERROR printed beside it. This is because the
receipt of the data is not being acknowledged by any
device on the bus (certainly not the ELM327, as it is by
default a completely silent monitor). See our ‘AN05 Bench Testing OBD Interfaces' application note for
more information on this, and some advice on what to
do. If you have a v1.4b chip, you do not have to take
special measures, however, simply turn off the silent
monitoring with:
to select a long timeout. You will not do any harm if
you set the timeout with a v1.4b chip, but the timing
will not be optimal, as you will stop the ELM327 from
varying the setting based on the type of message
being received.
The new ELM327 v1.4b also offers one other
variation on the timer setting - the ability to extend the
AT ST time by switching a x5 timer multiplier on and
off (see the JTM5 command). This may be useful
when requesting data that will have a multiline
response while similar data is already flowing. Since
there can be only one message like this at a time on
the bus, the response to your request would have to
and there should be no more RX ERRORs. Once you
have this sorted out, repeat the request. If all goes
well, you should see several replies, similar to this:
00 FF 00 00 00 00 FF FF
00 FF 00 00 00 00 FF FF
You will likely need to stop the flow of data by
pressing any key on the keyboard. This is because the
DM1 command is actually a special form of a
monitoring command, and all monitoring needs to be
stopped by the user. The response means that there
are currently no active trouble codes, by the way.
To see the exact same response, you can also
Elm Electronics – Circuits for the Hobbyist
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Using J1939 (continued)
Monitor for PGN 00FECA (which is the code for DM1):
Note that the ELM327 requires that you send hex
digits for all data, as shown above (and as used by all
other protocols). Many of the PGN numbers are listed
in the J1939 standard as both a decimal and a hex
number, so choose the hex version.
You will likely find in your testing that the PGNs
you encounter often begin with a 00 byte as above. To
simplify matters for you, the ELM327 has a special
version of the MP command that will accept a four digit
PGN, and assumes that the missing byte should be
00. An equivalent way to ask for 00FECA is then:
which is a little more convenient.
One new feature of the ELM327 v1.4b is the ability
to tell the IC how many messages to retrieve when
monitoring for PGNs. For example, to see only two
responses to the MP FECA command, send:
This saves having to send a character to stop the
flow of data, and also is very convenient when dealing
with multiline messages. While the standard OBD
requests allow you to define how many frames (ie
lines) of information are to be printed with a similar
single digit, the single digit with the MP command
actually defines how many complete messages to
obtain. For example, if the DM1 message is 33 lines
long, then sending AT MP FECA 1 will cause the
ELM327 to show all 33 lines, then stop monitoring and
print a prompt character.
By default, all J1939 messages have the ‘header’
information hidden from view. In order to see this
information (actually the ID bits), you will need to turn
the header display on:
>AT H1
A single response to FECA might then look like:
6 0FECA 00 00 FF 00 00 00 00 FF FF
Notice that the ELM327 separates the priority bits
from the PGN information. The ELM327 also uses only
one digit to represent the two extra PGN bits, both of
which may seem unusual if you are used to different
software. We find this a convenient way to show the
actual J1939 information in the header.
If you prefer to see the ID bits separated into bytes
instead, simply turn off the J1939 header formatting
Repeating the above request would then result in
a response of this type:
18 FE CA 00 00 FF 00 00 00 00 FF FF
The differences are clearly seen. If displaying the
information in this manner, remember that the first
‘byte’ shown actually represents five bits, and of them,
the leftmost three are the priority bits.
The MP command is very useful for getting
information in a J1939 system, but not all information
is broadcast. Some information must be obtained by
making a query for it. Just like the other OBD requests
where you specify the information that you need (with
a mode and a PID), to make a query in a J1939
system, you provide the PGN number and the system
responds with the required data.
For example, to request the current value of the
engine coolant temperature (which is part of PGN
00FEEE), you would send a request for PGN 00FEEE,
and extract the data. To do this, send:
to which you might receive:
if the headers were on. Note that if you request a PGN
that is already being broadcast, you may very well
receive many replies, as the ELM327 configures itself
to receive anything that is related to the PGN
If you are familiar with the J1939 standard, you will
be aware that it actually specifies a reverse order for
the sending of the data bytes of a PGN request. That
is, the data bytes for the above request are actually
sent as EE FE 00, and not as 00 FE EE. Since it can
be very confusing to have to reverse some numbers
and not others, the ELM327 automatically handles this
for you, reversing the bytes provided. In this way, you
can directly request PGNs using numbers as they are
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Using J1939 (continued)
written on the page (if they are written as hex digits),
and the ELM327 will make it work for you. If you do not
want the ELM327 to alter the byte order, the feature
can be disabled (by sending an AT JS command).
The ELM327 always assumes that when you start
making requests of this type, you do not know what
devices are connected to the J1939 bus. That is, by
default the ELM327 sends all requests to the 'global
address' (ie all devices), and then looks for replies.
Often, this works well, but J1939 devices are not
required to respond to such general inquiries, and may
not if they are busy. For this reason, it is usually better
to direct your queries to a specific address, once it is
In order to determine the address to send to, you
may have to monitor the information on the bus for a
while. Make sure that the headers (ID bits) are being
displayed, and note what is shown in the Source
Address position, which is immediately before the data
bytes. In the previous example, this would be 00
(which J1939 defines as the address for engine #1).
As an example, let us assume that it is engine #1 that
you wish to direct your queries to. To do this, you will
want to change the Destination Address from FF (the
global address) to 00 (engine #1).
By default, the ELM327 uses 6 0EAFF F9 for the
ID bits of all requests (or 18 EA FF F9 if you prefer).
That is, it uses a priority of 6, to make a request (EA)
to the global address (FF) by the device at F9 (the
scan tool). Since you only wish to alter the EAFF F9
portion of the ID bits and not the priority, you may do
this with the set header command:
>AT SH EA 00 F9
The priority bits rarely need to be changed, but if
you do need to change them, it is done with the CAN
Priority (AT CP) command.
After making the above change, all data requests
will be directed to the engine, so don’t forget to change
the headers if you wish to again make global requests.
Note that the AT SH command allows you to change
the source (or tester) address at will, so be careful with
this as addresses should really be negotiated using
the method described in J1939-81 and you might
conceivably choose an address that is already in use.
The current version of the ELM327 does not support
J1939-81 address negotiation, so can not obtain an
address for you.
Once the ELM327 has been configured to send all
messages to address 00, repeat the request:
6 0E8FF 00 01 FF FF FF FF EE FE 00
This response is of the ‘acknowledgement’ type
(E8), which is being broadcast to all (FF) by the device
with address 00. The last three data bytes show the
PGN requested, in reverse byte order, so we know this
is a response to our request. Looking at the other data
bytes, the first is not 00 (which we would expect for a
positive acknowledgement), it is 01 which means
negative acknowledgement. Since all requests to a
specific address must be responded to, the device at
address 00 is responding by saying that it is not able
to respond. That is, retrieve the information using the
MP command.
If the ECU had been able to reply to the request,
the format of the response would have been slightly
different. For example, if a request for engine run time
(PGN 00FEE5) had been made, the response might
have been like this:
6 0FEE5 00 80 84 1E 00 FF FF FF FF
Notice that the PGN appears in the header for
these types of replies, and the data bytes are those
defined for the SPNs in the PGN.
All responses to a request are printed by the
ELM327, whether they are a single CAN message, or
a multisegment transmission as defined by the
transport protocol (J1939-21). If the responses are
multisegment, the ELM327 handles all of the
negotiation for you. As an example, a multisegment
response to a DM2 request might look like this:
7 0EBF9 00 01 04 FF 50 00 04 0B 54
7 0EBF9 00 02 00 00 01 5F 05 02 31
7 0EBF9 00 03 6D 05 03 03 FF FF FF
if the headers are on, and would appear as:
01: 04 FF 50 00 04 0B 54
02: 00 00 01 5F 05 02 31
03: 6D 05 03 03 FF FF FF
if the headers are off. Note that multiframe messages
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Using J1939 (continued)
always send eight bytes of data, and fill in unused byte
positions with FFs.
With the headers off, the multiline response looks
very similar to the multiline responses for ISO15765-4.
The first line shows the total number of bytes in the
message, and the other lines show the segment
number, then a colon, and the data bytes following.
Note that the byte count is a hexadecimal value (ie the
‘012’ shown means that there are 18 bytes of data).
The one line that shows the total number of data
bytes is actually called a ‘Connection Management’ or
‘TP.CM’ message. It has a specific format, but the only
bytes that are typically relevant are those that provide
the total message size in bytes. In order to see the
other bytes, you must turn CAN Auto Formatting off
(AT CAF0), and then repeat the request. Note that this
will only show the entire TP.CM message if you have
an ELM327 v1.4b, and not a previous version.
This has been a brief description of how to use
the ELM327 in a typical J1939 environment. If you can
monitor for information, make global requests as well
as specific ones, and receive single or multiframe
responses, then you have the tools necessary to at
least diagnose most vehicle problems.
The FMS Standard
Several European heavy duty truck and bus
manufacturers have joined to form an organization for
standardizing the way in which information is retrieved
from these large vehicles. The result of their work is
the FMS (or Fleet Management Systems) Standard,
and the Bus-FMS Standard.
The FMS standard is based on a subset of the
250 kbps J1939 protocol, which uses only broadcast
messages for the information. In order to not
compromise the integrity of the vehicle’s CAN bus, the
standard also specifies a gateway device to provide
separation between (potentially unskilled) users and
the critical control information on the vehicle.
The information contained in the FMS messages
is defined by PGNs, using the same PGN numbers as
for J1939. The difference is that they only define a
small subset of those specified by J1939.
To monitor the information provided by an FMS
gateway, simply use the AT MP command with the
appropriate PGN number. We should caution that
some information (VIN, software version, etc.) is only
transmitted every 10 seconds, so some patience is
required when waiting for the data.
The FMS standard is completely open, and still
evolving (the latest update was September 2010). For
more information, visit the web sites:
FMS Standard
Bus FMS Standard
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Programmable Parameters
The ELM327 contains several programmable
memory locations that retain their data even after
power is turned off. Every time the IC is powered up,
these locations are read and used to change the
default settings for such things as whether to display
the headers, or how often to send ‘wakeup’ messages.
The settings, or parameters, can be altered by the
user at any time using a few simple commands. These
Programmable Parameter commands are standard AT
Commands, with one exception: each one requires a
two-step process to complete. This extra step provides
some security against random inputs that might try to
make changes.
The following pages list the currently supported
Programmable Parameters for this version of the
ELM327. As an example of how to use them, consider
PP 01 (shown on page 55) which sets the default state
for the AT H command. If you are constantly powering
your ELM327 and then using AT H1 to turn the
headers on, you may want to change the default
setting, so that they are always on by default. To do
this, simply set the value of PP 01 to 00:
This changes the value associated with PP 01, but
does not enable it. To make the change effective, you
must also type:
>AT PP 01 ON
At this point, you have changed the default setting
for AT H1/H0, but you have not changed the actual
value of the current AT H1/H0 setting. From the ‘Type’
column in the table on page 55, you can see that the
change only becomes effective the next time that
defaults are restored. This could be from a reset, a
power off/on, or possibly an AT D command.
With time, it may be difficult to know what changes
you have made to the Programmable Parameters. To
help with that, the ELM327 provides a Programmable
Parameter Summary (PPS) command. This simply
prints a list of all of the supported PPs, their current
value, and whether they are on/enabled (N), or
off/disabled (F). For an ELM327 v1.4b IC, with only the
headers enabled (as discussed above), the summary
table would look like this:
01:00 N
You can see that PP 01 now shows a value of 00,
and it is enabled (oN), while the others are all off.
Another example shows how you might change
the CAN filler byte. Some systems use ‘AA’ as the
value to put into unused CAN bytes, while the ELM327
uses ‘00’ by default. To change the ELM327’s
behaviour, simply change PP 26:
>AT PP 26 ON
>AT PP 01 SV 00
00:FF F
02:FF F
03:32 F
Again, PP 26 is of type ‘D’, so the above change
will not actually take effect until the AT D command is
issued, or the ELM327 is reset.
The Programmable Parameters are a great way to
customize your ELM327 for your own use, but you
should do so with caution if using commercial
software. Most software expects an ELM327 to
respond in certain ways to commands, and may be
confused if the carriage return character has been
redefined, or if the CAN response shows data length
codes, for example. If you make changes, it might be
best to make small changes and then see the effect of
each, so that it is easier to retrace your steps and
‘undo’ what you have done. If you get in too deeply,
don’t forget the ‘all off’ command:
No matter what software you use, you might get
into more serious trouble, should you change the baud
rate, or the Carriage Return character, for example,
and forget what you have set them to. The Carriage
Return value that is set by PP 0D is the only character
that is recognized by the ELM327 as ending a
command, so if you change its value, you may not be
able to undo your change. In this case, your only
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Programmable Parameters (continued)
recourse may be to force all of the PPs off with a
hardware trick.
When the ELM327 first powers up, it looks for a
jumper between pin 28 (the OBD Tx LED output) and
circuit common (V SS). If a jumper is in place, it will turn
off all of the PPs for you, restoring the IC to the factory
defaults. To use this feature, simply connect a jumper
to circuit common (which appears in numerous places
- pins 8 or 19 of the ELM327, pin 5 of the RS232
connector, one end of most capacitors, or at the OBD
connector), then hold the other end of the jumper to
pin 28 while turning the power on. When you see the
RS232 Rx LED begin to flash quickly, remove the
jumper – the PPs are off.
This feature should only be used when you get
into trouble too deeply, and it’s your only choice (since
putting jumpers into a live circuit might cause damage
if you put it into the wrong place). As well, it is only
available beginning with version 1.2 of the IC, and can
not be used with any previous versions.
Programmable Parameter Summary
The following pages provide a list of the currently
available Programmable Parameters. Note that the
‘Type’ column indicates when changes will take effect.
Possible values are:
I - the effect is Immediate,
R - takes effect after a Reset
(AT Z, AT WS, MCLR or power off/on)
P - needs a Power off/on type reset
(AT Z, MCLR, or power off/on)
D - takes effect after Defaults are restored
(AT D, AT Z, AT WS, MCLR or power off/on)
Perform AT MA after powerup or reset
Printing of header bytes (AT H default setting)
Allow long messages (AT AL default setting)
00 = ON
00 = ON
00 = ON
NO DATA timeout time (AT ST default setting)
setting = value x 4.096 msec
00 to FF
Default Adaptive Timing mode (AT AT setting)
00 to 02
OBD Source (Tester) Address. Not used for J1939 protocols.
00 to FF
Last Protocol to try during automatic searches
01 to 0C
Character echo (AT E default setting)
00 = ON
00 to FF
Linefeed Character
(205 msec)
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Programmable Parameter Summary (continued)
RS232 baud rate divisor when pin 6 is high (logic 1)
08 to FF
The baud rate (in kbps) is given by 4000 ÷ (PP 0C value)
For example, 500 kbps requires a setting of 08 (since 4000/8 = 500)
Here are some example baud rates, and the divisor to be used:
Baud Rate
PP 0C value
Note that the PP 0C value must be provided as hex digits only. The
decimal values (in brackets) are only shown for your convenience.
Carriage Return Character
used to detect and send line ends
00 to FF
Power Control options
00 to FF
Each bit of this byte controls an option, as follows:
b7: Master enable
0: off
1: on
if 0, pins 15 and 16 perform as described for v1.0 to v1.3a
must be 1 to allow any Low Power functions
b6: Pin 16 full power level
0: low
1: high
normal output level, is inverted when in low power mode
b5: Auto LP control
0: disabled
1: enabled
allows low power mode if the RS232 activity stops
b4: Auto LP timeout
0: 5 mins
no RS232 activity timeout setting
1: 20 mins
b3: Auto LP warning
0: disabled
1: enabled
if enabled, says ‘ACT ALERT’ 1 minute before RS232 timeout
b2: Ignition control
0: disabled
1: enabled
allows low power mode if the IgnMon input goes low
b1: Ignition delay
0: 1 sec
1: 5 sec
delay after IgnMon (pin 15) returns to a high level, before
normal operation resumes
b0: reserved for future - leave set at 0
J1850 voltage settling time
setting = value x 4.096 msec
00 to FF
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56 of 76
Programmable Parameter Summary (continued)
J1850 Break Signal monitor enable
(reports BUS ERROR if break signal duration limits are exceeded)
J1850 Volts (pin 3) output polarity
normal = Low output for 5V, High output for 8V
invert = High output for 5V, Low output for 8V
00 = ON
00 = invert
FF = normal
Auto search time delay between protocols 1 & 2
setting = value x 4.096 msec
00 to FF
Minimum inter-message time (P2) for protocols 3 to 5
setting = value x 2.2 msec
00 to FF
Default ISO baud rate (AT IB default setting)
00 = 96
FF = 10
(22 msec)
00 to FF
Auto search time delay between protocols 4 & 5
setting = value x 4.096 msec
00 to FF
Time delay after protocol 5 attempt during an automatic search,
but only if protocols 3 & 4 have not yet been tried.
setting = value x 20.48 msec
00 to FF
Default CAN Silent Monitoring setting (for AT CSM)
00 = OFF
00 = ON
00 = ON
CAN auto flow control (AT CFC default setting)
(205 msec)
ISO wakeup message rate (AT SW default setting)
setting = value x 20.48 msec
CAN auto formatting (AT CAF default setting)
(2.99 sec)
(no delay)
(819 msec)
CAN filler byte (used to pad out messages)
00 to FF
Printing of the CAN data length (DLC) when printing header bytes
(AT D0/D1 default setting)
00 = ON
CAN Error Checking (applies to protocols 6 to C)
00 to FF
Each bit of this byte controls an option, as follows:
b7: ISO15765 Data Length
0: accept any 1: must be 8 bytes
b6: ISO15765 PCI=00
0: allowed
1: not allowed
b5: Search after ERR94
0: normal
1: CAN is blocked
b4: Search after LV RESET
0: normal
1: CAN is blocked
b3: Wiring Test
0: bypass
1: perform
b2: reserved for future - leave set to 0
b1: reserved for future - leave set to 0
b0: reserved for future - leave set to 0
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Programmable Parameter Summary (continued)
Protocol A (SAE J1939) CAN baud rate divisor.
01 to 40
(250 Kbps)
The protocol A CAN baud rate is determined by this value
rate (in kbps) = 500 ÷ value
For example, setting this PP to 19 (ie. decimal 25) provides
a baud rate of 500/25 = 20 kbps.
Protocol B (USER1) CAN options.
00 to FF
Each bit of this byte controls an option, as follows:
b7: Transmit ID Length
0: 29 bit ID
1: 11 bit ID
b6: Data Length
0: fixed 8 byte 1: variable DLC
b5: Receive ID Length
0: as set by b7 1: both 11 and 29 bit
b4: baud rate multiplier
0: x1
1: x 8/7
(see note 3)
b3: reserved for future - leave set at 0.
b2, b1, and b0 determine the data formatting options:
b2 b1 b0
Data Format
ISO 15765-4
SAE J1939
Other combinations are reserved for future updates – results will
be unpredictable if you should select one of them.
Protocol B (USER1) baud rate divisor. See PP 2B for a description.
01 to 40
(125 Kbps)
Protocol C (USER2) CAN options. See PP 2C for a description.
00 to FF
Protocol C (USER2) baud rate divisor. See PP 2B for a description.
01 to 40
(50 Kbps)
1. The ELM327 does not accept decimal digits for the Programmable Parameters - all values are hexadecimal.
2. Several Programmable Parameters describe options in terms of bits. For all of them, b7 is the msb, and b0
is the lsb. As an example, the PP 2C default value of E0 can be shown as 11100000 in binary. This number
has has b7, b6 and b5 set to 1’s, while b4 to b0 are all 0’s.
3. When b4 of PP 2C or PP 2E are set, the CAN baud rate will be increased by a factor of 8/7, but the baud
rate displayed by the AT DP command will still show the base rate (as set by PP 2D or PP 2F). For example,
if you set PP 2C b4 to 1, and then PP 2D to 06, the base frequency will be 83.3 kbps. The AT DP command
will report 83 kbps, but the actual baud rate will be 83.3x8/7 = 95.2 kbps.
4. The descriptions for PP 0E bits 3 and 4 were accidentally reversed in the previous data sheet (document
ELM327DSG), and have been corrected.
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Using Higher RS232 Baud Rates
The RS232 serial interface has been maintained
throughout the ELM OBD products, largely due to its
versatility. Older computers, microprocessors and
PDAs can use it directly, as can USB, Bluetooth,
ethernet and wifi devices. It is simply one of the most
versatile interfaces available.
Most people will construct their ELM327 circuits
with an RS232 interface, mainly because it is relatively
easy and inexpensive to do. A circuit such as the one
shown in Figure 9 requires very few components and
works extremely well at speeds of up to 57600 bps.
Depending on your interface’s RS232 voltages, your
wiring practices, and your choice of components, it
may also work well at speeds as high as 115200 bps,
but that is the approximate limit of such a circuit, and
any design using this speed should be thoroughly
Users that would like to operate at speeds in the
range of 115200 bps or higher may wish to look at
some of the single IC solutions that are available.
These include devices such as the ADM232A from
Analog Devices (http://www.analog.com/), or the
popular MAX232 series of ICs from Maxim Integrated
Products (http://www.maxim-ic.com/). These are all
excellent devices that can be used for speeds of up to
115.2 kbps. We do caution that many of these types of
devices are only rated for operation up to 120 kbps,
however, so may not be suitable for higher data rates be sure to check the manufacturers data sheet before
committing to a design.
An RS232 interface needs relatively large voltage
swings, which are difficult to maintain at higher rates
with large cable capacitances to contend with. (A
typical interface is often limited to about 230.4 kbps
under ideal conditions.) If you need to operate the
ELM327 at these speeds or higher, it is recommended
that you consider alternatives.
One popular alternative is a USB data connection.
The USB interface is capable of very high data transfer
rates, certainly much higher than the 500 kbps limit of
the ELM327. Several manufacturers offer special
‘bridge’ circuits that simplify connecting an RS232
device (such as the ELM327) directly to the USB bus.
Examples are the CP2102 from Silicon Labs
(http://www.silabs.com/) or the FT232R from Future
Technology Devices (http://www.ftdichip.com/). If
planning to use the higher baud rates (ie up to 500
kbaud), these interfaces are essential.
We are often asked if it is possible to use a direct
connection to a microprocessor. That is certainly an
option, and one that allows a full speed connection at
essentially zero cost. If you are developing such an
interface, refer to page 64 for more details.
The default configuration for the ELM327 provides
an RS232 data rate of either 9600 baud, or 38400
baud, depending on the voltage level at pin 6 during
power up or reset. While the 9600 baud rate is not
adjustable, the 38400 one is (beginning with v1.2 of
the IC). There are two ways that the rate can be
changed – either permanently with a Programmable
Parameter, or temporarily with an AT command.
Programmable Parameter ‘0C’ is the memory
location that allows you to store a new baud rate which
replaces the 38.4 kbps high speed rate. The value is
stored in ‘non-volatile’ memory (EEPROM) that is not
affected by power cycles or resets (but changing this
value may affect the operation of some software
packages, so be careful how you use it).
If you store a new value in PP 0C, then enable it,
and if pin 6 is at a high level during the next powerup,
then your stored rate will become the new data rate. (If
it has not been enabled, the rate will revert to the
factory default of 38.4 kbps.) As an example, perhaps
you would like to have the ELM327 use a baud rate of
57.6 kbps, rather than the factory setting of 38.4 kbps.
To do this, determine the required value for PP 0C,
store this value in PP 0C, and then enable the PP.
The value stored in PP 0C is actually an internal
divisor that is used to determine the baud rate (it will
be 4000 kbps divided by the value of PP 0C). To
obtain a setting of 57.6, a baud rate divisor of 69 is
required (4000/69 is approximately 57.6). Since 69 in
decimal is 45 in hexadecimal, you need to tell the
ELM327 to set the value of PP 0C to 45, with this
>AT PP 0C SV 45
then enable the new value for use:
from that point (until PP 0C is turned off), the default
data rate will be 57.6K, and not 38.4K. Note that the
value that you write does not become effective until
the next full reset (a power off/on, AT Z, or MCLR
If you are designing your own circuitry, you will
know what your circuit is capable of, and can assign a
value to PP 0C. Software developers will not usually
know what hardware is to be connected, however, so
will not know what the limitations are. For these users,
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Using Higher RS232 Baud Rates (continued)
we have provided the BRD command.
This command allows a new baud rate divisor to
be tested, and then accepted or rejected depending on
the results of the test. See the chart at the right, which
shows how the command works.
As can be seen, the software first makes a request
for a new baud rate divisor, using this AT command.
For example, to try the 57.6K rate that was previously
discussed, the controlling PC would send:
Request for a new
baud rate divisor:
ELM327 responds
with ‘OK’
(if it is supported)
If the ELM327 is an older version, it will not
support this command, and will return with the familiar
‘?’ character. If it does support the command, it will
respond with ‘OK’, so the software knows whether to
proceed or not. No prompt character follows the ‘OK’
reply; it is followed only by a carriage return character
(and optionally, a linefeed character).
Having sent an ‘OK’, the ELM327 then switches to
the new (proposed) baud rate, and then simply waits a
predetermined time (nominally 75 msec). This period
is to allow the PC sufficient time to change its baud
rate. When the time is up, the ELM327 then sends the
ID string (currently ‘ELM327 v1.4b’) to the PC at the
new baud rate (followed by a carriage return) and
waits for a response.
Knowing that it should receive the ELM327 ID
string, the PC software compares what was actually
received to what was expected. If they match, the PC
responds with a carriage return character, but if there
is a problem, the PC sends nothing. The ELM327 is
meanwhile waiting for a valid carriage return character
to arrive. If it does (within 75 msec), the proposed
baud rate is retained, and the ELM327 says ‘OK’ at
this new rate. If it does not see the carriage return, the
baud rate reverts back to the old rate. Note that the PC
might correctly output the carriage return at this new
rate, but the interface circuitry could corrupt the
character, and the ELM327 might not see a valid
response, so your software must check for an ‘OK’
response before assuming that the new rate has been
Using this method, a program can quickly try
several baud rates, and determine the most suitable
one for the connected hardware. The new baud rate
will stay in effect until reset by an AT Z, a Power
Off/On, or a MCLR input. It is not affected by the AT D
(set Defaults), or AT WS (Warm Start) commands.
Program switches to
the new baud rate,
and waits for input
ELM327 switches to
new baud rate and
waits for 75 msec*
ELM327 sends
the AT I string
If the Rx is good,
Program sends a
carriage return
ELM327 waits
up to 75 msec*
for a carriage return
Baud rate reverts
to the previous
baud rate
Elm Electronics – Circuits for the Hobbyist
ELM327 says ‘OK’
(and remains at the
new baud setting)
Print a prompt,
and wait for the
next command
* the 75 msec time is adjustable
with the AT BRT hh command
60 of 76
Setting Timeouts - AT ST and AT AT Commands
Users often ask about how to obtain faster OBD
scanning rates. There is no definite answer for all
vehicles, but the following information may help with
understanding how the settings might apply to your
A typical vehicle request and response is shown in
the diagram below:
request is sent
a value that should work for most situations. It is
enabled by default, but can be disabled with the AT0
command should you not agree with what it is doing
(there is also an AT2 setting that is a little more
aggressive, should you wish to experiment). For 99%
of all vehicles, we recommend that you simply leave
the settings at their default values, and let the ELM327
make the adjustments for you.
Consider the following times taken from a J1850
VPW vehicle, in response to an 01 00 request:
4 msec
58 msec
ELM waits up
to 200 msec
ELM waits 200 msec
for more responses
The ELM327 sends a request then waits up to
200 msec for a reply. If none were to come, an internal
timer would stop the waiting, and the ELM327 would
print ‘NO DATA’.
After each reply has been received, the ELM327
must wait to see if any more replies are coming (and it
uses the same internal timer to stop the waiting if no
more replies arrive). With our initial OBD products (the
ELM320, ELM322 and ELM323) we found that older
vehicles often needed a timeout setting of about 100
msec, and occasionally needed more, so we settled on
a standard default setting of 200 msec.
If a typical vehicle query response time were about
50 msec, and the timeout were set to 200 msec, the
fastest scan rate possible would only be about 4
queries per second. Changing the ST time to about
100 msec would almost double that rate, giving about
7 queries per second. Clearly, if you were to know how
long it takes for your vehicle to reply, you might be
able to improve on the scan rate, by adjusting the ST
It is not easy to tell how fast a vehicle replies to
requests. For one thing, requests all have priorities
assigned, so responses may be fast at some times,
and slower at others. The physical measurement of
the time is not easy either - it requires expensive test
equipment just to make one measurement. To help
with this, we added a new feature to the v1.2 IC, called
Adaptive Timing.
Adaptive Timing actually makes the response time
measurements for you, and adjusts the AT ST time to
(ECU 10)
(ECU 18)
The engine controller responds very quickly, but
the transmission takes considerably longer. The
adaptive timing algorithm measures the longer
transmission response times and will use them to set
the timeout, likely to a value in the range of 90 msec.
With a timeout of 90 msec, the maximum scan rate
would be about 6 readings per second.
Surely there has to be a way to eliminate that final
timeout, if you know how many responses to expect?
There is a way, as of v1.3 of the ELM327.
Instead of sending 01 00 for the above request,
the ELM327 will now also accept 01 00 2. This tells the
IC to send 01 00, then return immediately after
receiving 2 responses. It can not speed up a slow
ECU, but it will eliminate the final delay, as the
ELM327 knows the number of responses to expect.
This one change might give you 10 to 12 responses
per second, instead of the 6 obtained previously.
We do caution that you use this new feature
carefully. If you set the last digit to a number that is
less than the actual number of responses, then
acknowledgements that may be required will not be
sent, and some protocols may begin resending the
message, looking for a response. This will lead to
unnecessary network congestion, which must be
avoided. Before using this feature, always determine
the number of responses that will be coming from the
vehicle, and then set the responses digit to that value.
Elm Electronics – Circuits for the Hobbyist
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Power Control
Often, the ELM327 is connected to a vehicle for
only a short time, so power consumption is not of great
concern. Occasionally, the ELM327 may be connected
for longer times, however, possibly without the engine
running. For these applications, it is often desirable to
be able to put the circuit into a low power ‘standby’
state, and have it return to normal operation when
needed. The new power control features of the
ELM327 were introduced for this.
There are three ways in which the ELM327 can be
placed into the low power standby mode (shown
pictorially in Figure 6 below). None of the three will
work without having the master enable (ie bit 7 of
PP 0E) set to ‘1’, which it is by default.
The first method is with an AT command. You may
simply send:
will reverse its state. The ELM327 will then reduce its
power level, and begin monitoring for inputs that would
cause a shift back to full power.
The next method allows automatic switching to the
low power mode when there has been no RS232 input
for a period of time - a good backup for your low power
strategy. To enable this method, both b7 and b5 of
PP 0E must be set to ‘1’. The time delay (either 5 or 20
minutes) is set by b4, and the printing of a warning is
enabled with b3. The warning is handy in some cases
– it is the activity alert message (‘ACT ALERT’) and is
sent 1 minute before the timer is about to time out.
When the timer does time out, you will see a low
power alert warning (‘LP ALERT’), and then 2 seconds
later, all of the outputs will change as described above
for the AT LP command.
The final method to enter the low power mode is
by a low level appearing at the ignition monitor input
(pin 15 - IgnMon), when both b7 and b2 of PP 0E are
set to ‘1’, allowing it.
The ignition monitor method uses a short internal
delay (‘debounce’) to be sure that the low level is a
legitimate ‘key off’ and not just some noise. After it is
sure, the ELM327 will then send a low power alert
message (‘LP ALERT’), and 2 seconds later it will
and the IC will go to the low power mode after a one
second delay (which allows the controlling circuit a
little time to perform some housekeeping tasks).
At the end of the delay, all of the ELM327 outputs
go to their recessive/off state, the pin 3 control for the
J1850 voltage will go low, and the pin 16 control output
1 sec
Go to Low Power
• pin 16 = b6
• LEDs off
• pin 3 = 0V
• µP to low power
idle (at prompt),
waiting for input
2 sec
5 min or 20 min
time delay on pick up
(fast reset)
RS232 Rx
(pin 18)
1 min
(pin 15)
Figure 6. Enabling the Low Power Mode
Elm Electronics – Circuits for the Hobbyist
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Power Control (continued)
switch all of the outputs as was described for the other
When connecting to pin 15, care should be taken
to not pass excessive current (>0.5 mA) through the
protection diodes, and the circuit should also provide
some filtering for ignition noise. Typically a circuit like
this works well (note that the Schmitt input on pin 15
allows the use of large value capacitors):
+12V switched
by the ignition
The new AT IGN command can always be used to
read the level at pin 15, regardless of the setting of the
PP 0E enable bits. This may be used to advantage if
you wish to manually shut down the IC, using your
own timing and criteria. Recall that the alternate
function for pin 15 is the RTS input which will interrupt
any OBD processing that is in progress. If the
ELM327 reports an interrupt with the ‘STOPPED’
message, you can then check the level at pin 15 with
the AT IGN command, and make your own decisions
as to what should be done. For that matter, you don’t
even need to reduce the power based on the input you might possibly do something entirely different.
Having put the ELM327 into Low Power mode,
you will need a method to wake it up. This is done by
RS232 Rx
(pin 18)
‘interrupting’ the IC in ways that are very similar to
what is done while monitoring. Refer to Figure 7 below
for a pictorial view of these methods.
Either a low level pulse at the RS232 Rx input, or
a low then high level at the IgnMon input will cause the
ELM327 to return to full power operation, and perform
a Warm Start reset. Note that PP 0E bit 2 does not
have to be set for the IgnMon to wake the circuit - the
ELM327 always monitors this pin, and will wake the
circuit after the delay that is set by PP 0E bit 1.
The RS232 input is not as sensitive as normal
when in the low power mode. For this reason, the
RS232 input pulse to wake the system must be at
least 128 usec wide. This is easily accomplished by
sending a space or @ character if the baud rate is less
than about 57.6 kbps, but when you use higher baud
rates, you may have trouble. For higher baud rates,
consider temporarily shifting to a lower baud rate, or
see if your software can generate a ‘break’ signal. If
you are directly connected to a microprocessor, then
you might want to generate your own break signal in
This has discussed some of the software aspects
of using the new Power Control feature. Refer to the
‘Modifications for Low Power Standby Operation’
section (page 70) for a discussion of some of the
electrical design considerations.
Figure 7. Returning to Normal Operation
128 µsec min
pulse width
(pin 15)
1 sec or 5 sec
time delay on pick up
(fast reset)
1 sec
perform a
warm start
Go to Full Power
rising edge ( )
was detected
• µP to normal
• pin 16 = b6
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Microprocessor Interfaces
A very common question that we receive is ‘Can I
connect the ELM327 directly to my own circuit, or must
I use the RS232 interface shown?’ Certainly you may
connect directly to our ICs, and you do not need to use
an RS232 interface. There are a few items to consider,
The ELM327 is actually a microprocessor that
contains a standard UART type interface, connected to
the RS232 Tx and Rx pins. The logic type is CMOS,
and this is compatible with virtually all 5V TTL and
CMOS circuits, so you should be able to connect
directly to these pins provided that the two devices
share the same power supply (5V), and that they are
not physically more than about 10 to 20 inches apart
(CMOS circuits are subject to latchup from induced
currents, which may be a problem if you have long
The normal (idle) levels of the ELM327 transmit
and receive pins are at the VDD (5V) level. Most
microprocessors and RS232 interface ICs expect that
to be the idle level, but you should verify it for your
microprocessor before connecting to the ELM327. The
connections are straightforward - transmit connects to
receive, and receive connects to transmit, as shown
below. Don’t forget to set both devices to the same
baud rate.
The ELM327 also provides a hand-shaking feature
that may simplify the flow of data for you. The interface
consists of two pins - an input an an output. The input
is called ‘request to send’ (RTS), and it is used to
interrupt the ELM327, just the same as tapping a key
on the keyboard when using a terminal program. The
output pin (‘Busy’) is used by the ELM327 to tell your
system that it is processing data.
To use the handshaking feature, set one of your
port pins to normally provide a high output, and
connect it to the RTS input (pin 15). Use another port
pin as an input to monitor the ELM327 Busy output
(pin 16). When you want to send a command, simply
check the Busy output first. If it is at a high logic level,
then either wait for it to go low, or if you need to
interrupt the IC, then bring the RTS line low and wait
for the Busy line to go low. (You might want to
consider using an edge triggered interrupt on the Busy
output, if one is available). When Busy does go low,
restore your RTS line to a high level, and then send
your command to the ELM327. No need to worry
about the ELM327 becoming busy again after you
raise the RTS line at this point – once Busy goes low,
the ELM327 will wait (indefinitely) for your command.
If you do not use the RTS input on the ELM327, it
must be connected to a high logic level, as shown
your microprocessor
Busy RTS
The ELM327 and your
microprocessor must
use the same 5V supply
Elm Electronics – Circuits for the Hobbyist
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Example Applications
The SAE J1962 standard dictates that all OBD
compliant vehicles must provide a standard connector
near the driver’s seat, the shape and pinout of which is
shown in Figure 8 below. The circuitry described here
can be used to connect to this J1962 plug without
modification to your vehicle.
Figure 8. The J1962 Vehicle Connector
The male J1962 connector required to mate with a
vehicle’s connector may be difficult to obtain in some
locations, and you might be tempted to improvise by
making your own connections to the back of your
vehicle’s connector. If doing so, we recommend that
you do nothing that would compromise the integrity of
your vehicle’s OBD network. The use of any connector
which could easily short pins (such as an RJ11 type
telephone connector) is definitely not recommended.
The circuit on page 67 (Figure 9) shows how the
ELM327 might typically be used. Circuit power is
obtained from the vehicle via OBD pins 16 and 5 and,
after a protecting diode and some capacitive filtering,
is presented to a five volt regulator. (Note that a few
vehicles have been reported to not have a pin 5 – on
these you will use pin 4 instead of pin 5.) The regulator
powers several points in the circuit as well as an LED
(for visual confirmation that power is present). We
have shown a 78L05 for the regulator as that limits the
current available to about 100mA which is a safe value
for experimenting. The CAN interface is a low
impedance circuit however, and if doing sustained
transmissions on CAN, this type of regulator may
cause LV RESETs or possibly shut down on overtemperature. Should you experience either of these
problems, you may want to consider using a 1 Amp
version of the regulator (ie 7805).
The top left corner of Figure 9 shows the CAN
interface circuitry. We do not advise making your own
interface using discrete components – CAN buses
may have a lot of critical information on them, and you
can easily do more harm than good if you fail. It is
strongly recommended that you use a commercial
transceiver chip as shown. The Microchip MCP2551 is
used in our circuit, but most major manufacturers
produce CAN transceiver ICs – look at the NXP
PCA82C251 (NXP was formerly Philips), the Texas
Instruments SN65LBC031, and the Linear Technology
LT1796, to name only a few. Be sure to pay attention
to the voltage limits – depending on the application,
you may have to tolerate 24V, not just 12V.
The next interface shown is for the ISO 9141 and
ISO 14230 connections. We provide two output lines,
as required by the standards, but depending on your
vehicle, you may not need to use the ISO-L output.
(Many vehicles do not require this signal for initiation,
but some do, so it is shown here.) If your vehicle does
not require the L line, simply leave pin 22 unused.
The ELM327 controls both of the ISO outputs
through NPN transistors Q6 and Q7 as shown. These
transistors have 510Ω pullup resistors connected to
their collectors, as the standard requires. We are often
asked about substitutes for these resistors – if you
need to substitute, you can either go up to 560Ω or
possibly make 510Ω from two resistors (240Ω + 270Ω
1/4W resistors work well), but we do not recommend
using a lower value as it stresses every device on the
bus. Note that 1/2W resistors are specified in Figure
10 as a short at 13.8V causes about 0.4W dissipation.
Data is received from the K Line of the OBD bus
and connected to pin 12 after being reduced by the
R20/R21 voltage divider shown. Because of the
Schmitt trigger input on pin 12, these resistors will give
typical threshold levels of 9.1V (rising) and 4.7V
(falling), providing a large amount of noise immunity
while also protecting the IC. If you connect test
equipment in parallel with R21, it will cause these
thresholds to rise slightly, and you may begin to see
receive errors. Also, if you use a 9V test source, or the
vehicle’s battery voltage is low, you may also see
receive errors, or have problems connecting. If that
happens, we advise that you increase the value of R21
(33KΩ is a good value to start with).
The final OBD interface shown is for the two
J1850 standards. The J1850 VPW standard needs a
positive supply of up to 8V while the J1850 PWM
needs 5V, so we have shown a two level supply that
can provide both. This dual voltage supply uses a
317L adjustable regulator as shown, controlled by the
pin 3 output. With the resistor values given, the
selected voltages will be about 7.5V and 5V, which
works well for most vehicles. The two J1850 outputs
are driven by the Q1-Q2 combination for the Bus+,
and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider as
Elm Electronics – Circuits for the Hobbyist
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Example Applications (continued)
was used for the ISO input. Typical threshold voltages
with the resistors shown will be about 4.2V (rising) and
2.2V (falling). The J1850 PWM input is a little different
in that it must convert a differential input into a singleended one for use by the ELM327. In operation, Q4 is
actually used as the difference amplifier. The Q4-D3
series circuit sets a threshold voltage of about 1V (for
noise immunity), while R11 limits the current flow, and
R12 keeps Q4 off when the input is left open-circuited.
Resistor R36 has been added to the circuit of
Figure 9, to help turn transistor Q4 off more rapidly in
certain circumstances. The resistor is generally not
required, but it may be helpful if you are connected to
a very high capacitance J1850 VPW system then force
the ELM327 to operate in the J1850 PWM mode, and
experience some false BUS ERRORs. We show the
resistor as an option and leave the choice whether to
install it up to you.
The voltage monitoring circuitry for the AT RV
command is shown in this schematic connected to pin
2 of the ELM327. The two resistors simply divide the
battery voltage to a safe level for the ELM327, and the
capacitor filters out noise. As shipped, the ELM327
expects a resistor divider ratio as shown, and sets
nominal calibration constants assuming that. If your
application needs a different range of values, simply
choose the resistor values to maintain the input within
the specified 0-5V limit, and then perform an AT CV to
calibrate the ELM327 to your new divider ratio. The
maximum voltage that the ELM327 can show is 99.9V.
A very basic RS232 interface is shown connected
to pins 17 and 18 of the ELM327. This circuit ‘steals’
power from the host computer in order to provide a full
swing of the RS232 voltages without the need for a
negative supply. The RS232 pin connections shown
are for a standard 9 pin connector. If you are using a
25 pin one, you will need to compensate for the
differences. The polarity of the ELM327’s RS232 pins
is such that they are compatible with standard
interface ICs (MAX232, etc.), so if you should prefer
such an interface, you can remove all of the discrete
components shown and use one of those.
The four LEDs shown (on pins 25 to 28) have
been provided as a visual means of confirming circuit
activity. They are not essential, but it is nice to see the
visual feedback when experimenting.
Finally, the crystal shown connected between pins
9 and 10 is a standard 4.000MHz microprocessor type
crystal. The 27pF crystal loading capacitors shown are
typical only, and you may have to select other values
depending on what is specified for the crystal that you
use. The crystal frequency is critical to circuit operation
and must not be altered.
We often receive requests for parts lists to
accompany our Example Applications circuits. Since
this circuit is more complex than most, we have
named/numbered all of the components and provided
a summary parts list (see Figure 10). Note that these
are only suggestions for parts. If you prefer another
LED colour, or have a different general purpose
transistor on hand, etc., by all means make the
change. A quick tip for those having trouble finding a
0.3’ wide socket for the ELM327: many of the standard
14 pin sockets can be placed end-to-end to form one
0.3’ wide 28 pin socket. For more help with building
and testing the circuit, see our Application Note AN02 ELM327 Circuit Construction.
What if you only want to use one of the protocols?
What if you want to use a USB interface? These are
common questions that we receive, and both are
addressed in Figure 11.
There are a few single IC products on the market
that allow you to connect an RS232 system directly to
USB. We have shown the CP2102 by Silicon
Laboratories (http://www.silabs.com) in Figure 11, but
there are others available as well – Future Technology
Devices (http://www.ftdichip.com), for example,
produces several. These ICs provide a very simple
and relatively inexpensive way to ‘bridge’ between
RS232 and USB, and as you can see, require very few
components to support them.
If using the CP2102, we do caution that it is very
small and difficult to solder by hand, so be prepared
for that. Also, if you provide protection on the data
lines with transient voltage suppressors (TVS’s), be
careful of which ones you choose, as some exhibit a
very large capacitance and will affect the transmission
of the USB data. Note also that the circuit as
presented will operate at a 38400 bits per second rate.
If you want to take full advantage of the speed of the
USB interface, you will need to change PP 0C to
obtain a higher baud rate.
Considering the OBD protocol portions of the
circuits in Figures 9 and 11, the differences should be
very apparent. The unused protocols in Figure 11 have
simply had their outputs ignored (left open circuit), and
their inputs wired to logic levels as shown on page 5.
Note that CMOS inputs must never be left floating.
The circuit maintains the status LEDs, and the
J1850 Bus+ circuitry, but the majority of the rest has
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R16 2.2KΩ
R18 2.2KΩ
pin 12
4 (DTR)
6 (DSR)
7 (RTS)
8 (CTS)
* see text
R3 470Ω
1 (DCD)
5 (SG)
2 (RxD)
3 (TxD)
4.7KΩ Q1
Figure 9.
An OBD to RS232 Interpreter
R9 10KΩ
J1850 Bus +
J1850 Bus R15
pin 14
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Example Applications (continued)
been eliminated. The voltage switching circuitry has
been reduced to a single 8V regulator as well, since
there will be no need to switch to 5V. Note that pin 3
has been intentionally left open-circuited as it is not
required by the voltage regulator.
The first time that this circuit is used, it will likely be
set to protocol 0, the default ‘automatic search’ mode
of operation (as shipped from the factory). When you
connect it to the J1850 VPW vehicle, it will then
automatically detect the protocol, and if the memory is
enabled (as shown), J1850 VPW will then become the
new default, with no action required by you. This will
work well for most applications, but if the circuit is used
on a vehicle with the key off, for example, then it will
again go searching for a new protocol. In general, you
do not want this to happen every time. It may be only a
minor inconvenience to have to wait while the ELM327
determines that it is ‘UNABLE TO CONNECT’, but why
go through it if you do not have to? If you know that
you will be using the circuit in a J1850 VPW only
application (protocol 2) then you should issue the
command AT SP 2 the very first time that the circuit is
powered. From that point on, the ELM327 will remain
set for protocol 2, whether it fails to make a connection
or not.
Depending on the circumstances, you may be able
to simplify this circuit even further, by possibly using
the USB connection to obtain 5V for the ELM327,
rather than the 78L05 regulator shown. We caution
that some protocols (CAN for example), may draw
more current than your USB connection is able to
supply, so review this first.
This has provided two examples of how the
ELM327 integrated circuit might be used. Hopefully it
has been enough to get you started on your way to
many more. The following section shows how you
might be able to optimize these circuits to reduce
power consumption…
D1 = 1N4001
D2, D3, D4, D5 = 1N4148
L1, L2, L3, L4 = Yellow LED
L5 = Green LED
Q1, Q3, Q5, Q6, Q7, Q9 = 2N3904 (NPN)
Q2, Q4, Q8 = 2N3906 (PNP)
U1 = ELM327
U2 = MCP2551
U3 = 78L05 (5V, 100mA regulator)
U4 = 317L (adjustable 100mA regulator)
Resistors (1/8W or greater, except as noted)
R32, R33= 100 Ω
R5 = 240 Ω
R1, R2, R3, R4, R27, R28, R29, R30 = 470 Ω
R17, R19 = 510 Ω 1/2W
R16, R18 = 2.2 KΩ
R6, R7, R14, R15, R23, R26, R31 = 4.7 KΩ
R8, R9, R11, R13, R22, R24, R25, R35 = 10 KΩ
R10, R21, R36 = 22 KΩ
R20, R34 = 47 KΩ
R12 = 100 KΩ
Capacitors (16V or greater, except as noted)
C1, C5 = 0.1uF 50V
C2, C6, C7 = 0.1uF
C3, C4 = 27pF
C8, C9 = 560pF 50V
X1 = 4.000MHz crystal
RS232 Conn = DB9F
IC Socket = 28pin 0.3’ (or 2 x 14pin)
Figure 10. Parts List for Figure 9
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Example Applications (continued)
26 25
(type ‘B’
1 (+5)
2 (D-)
3 (D+)
4 (SG)
J1850 Bus +
Figure 11. An OBD (J1850 VPW) to USB Interpreter
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Modifications for Low Power Standby Operation
The ELM327 may be placed in a low power
standby mode in which it consumes very little current.
This will find its greatest use with semi-permanent
vehicle installations where you want the current
consumption to be as low as possible (ideally zero)
when the ELM327 is not needed.
The following uses the Example Application circuit
of Figure 9, and modifies it slightly in order to show a
few ways of further reducing current consumption. The
resulting circuit (Figure 12) is shown on the next page.
Note that portions of the circuit that are the same as
Figure 9 are shown in grey, while the changes that we
are suggesting are shown in black.
To know how effective the changes are, we need
a base current reading for what the entire circuit
consumes. With 12.0V applied as ‘Battery Positive’,
the measured current for Fig. 9 is:
base current = 31.9 mA
Without making any wiring changes, you can
reduce this current by sending the Low Power
command (AT LP). The resulting current is:
regulators that are using it. In the next step, we will
change U3 (a 78L05) to an LP2950ACZ-5.0G, and see
how effective that is. While the LP2950 is a good
choice for its lower quiescent current, it does suffer
from stability problems if you do not provide capacitive
loading as shown. Note that the 4.7uF capacitor is
tantalum, while the 2.2µF one is aluminum. At this
point it may also be useful to review our Application
Note ‘AN03 - ELM327 Low Voltage Resets’, as you
may want to use an even larger capacitor on the 5V
side. After this change:
current after mod #2 = 6.2 mA
That worked well. We’ll now change the 317L
regulator (U4) for an LP2951ACM, but leave it’s pin 3
solidly connected to circuit common. Again, the 4.7uF
tantalum capacitor is required for stability (and it helps
with transient capability too). Note that the circuit
shown needs the ELM327 to provide a high level at pin
3 for a 5V output, and a low for 8V, so an inversion is
needed. To do this, set PP 12 to 00 with:
>AT PP 12 SV 00
current after AT LP = 19.7 mA
This change represents almost all of the ELM327’s
current (it needs a little in order to stay in standby). But
where is the other current coming from? One obvious
load is the LED that shows that the power is on. The
other is the CAN transceiver, U2. By disconnecting the
common connections from R31 and R1, and then
returning both to pin 16 of the ELM327, we can switch
the current that these two use. With this change
(shown as modification #1), the LP current becomes:
current after mod #1 = 10.3 mA
If we continue to reduce the load currents, we can
quickly get to a point where any currents injected from
external sources (ie. through the protection diodes at
inputs such as pins 2 or 12) will become significant
compared to the load currents. If these currents should
exceed the load current, the Vdd voltage will rise and
damage might result. To prevent the voltage from
rising, we recommend that you add either a zener
diode or a transient voltage suppressor (TVS) as
shown, directly across the 5V supply. Suggestions for
devices to consider are the 1N5232B zener diode or
the SA5.0AG TVS.
There is a considerable current still flowing in the
circuit at this point, but it should mainly be the voltage
>AT PP 12 ON
Then reset the chip, and measure the new current
when in LP mode:
current after mod #3 = 1.2 mA
OK. Now we tie pin 3 of the LP2951 to pin 16 of
the ELM327, so that the regulator can be disabled
entirely. The LP current is:
current after mod #4 = 1.0 mA
Why is there still current flowing? This is due to a
number of things, some that you can change, others
which you can not. The MCP2551 and the ELM327
are not completely shut off, just in low power mode.
The LP2950 regulator is operating normally, as the
MCP2551 and the ELM327 need it. There are currents
flowing into pin 3 of the LP2951, and through both the
R34/R35 and R20/R21 pairs. All of these little currents
eventually add up (to the 1 mA).
These few changes have reduced the total power
from 383 mW to 12 mW - a considerable savings. We
leave any more improvements to you…
Elm Electronics – Circuits for the Hobbyist
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R16 2.2KΩ
3 (TxD)
R18 2.2KΩ
pin 12
1 (DCD)
6 (DSR)
7 (RTS)
4 (DTR)
2 (RxD)
5 (SG)
8 (CTS)
* see text
4.7KΩ Q1
Figure 12.
Modifications to reduce power
R9 10KΩ
J1850 Bus +
J1850 Bus R15
pin 14
Elm Electronics – Circuits for the Hobbyist
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Error Messages and Alerts
The following shows what the ELM327 will send to
warn you of a condition or a problem. Some of these
messages do not appear if using the automatic search
for a protocol, or if the Programmable Parameter bits
disable them.
This is the standard response for a misunderstood
command received on the RS232 input. Usually it is
due to a typing mistake, but it can also occur if you try
to do something that is not appropriate (eg. trying to do
an AT FI command if you are not set for protocol 5).
This message occurs as a warning that there has
been no RS232 activity (ie commands or messages
received) for either 4 or 19 minutes (depends on the
setting of PP 0E bit 4). It means that the IC will be
initiating a switch to the Low Power (standby) mode of
operation in 1 minute, unless RS232 characters are
received during that time. This warning is not provided
if PP 0E bit 3 is 0.
The ELM327 provides a 256 byte internal RS232
transmit buffer so that OBD messages can be received
quickly, stored, and sent to the computer at a more
constant rate. Occasionally (particularly with CAN
systems) the buffer will fill at a faster rate than it is
being emptied by the PC. Eventually it may become
full, and no more data can be stored (it is lost).
If you are receiving BUFFER FULL messages,
and you are using a lower baud data rate, give serious
consideration to changing your data rate to something
higher. If you still receive BUFFER FULL messages
after that, you might consider turning the headers and
maybe the spaces off (with AT H0, and AT S0), or
using the CAN filtering commands (AT CRA, CM and
CF) to reduce the amount of data being sent.
This occurs when the ELM327 tries to send a
message, or to initialize the bus, and detects too much
activity to do so (it needs a pause in activity in order to
insert the message). Although this could be because
the bus was in fact very busy, it is almost always due
to a wiring problem that is giving a continuously active
input. If this is an initial trial with your new ELM327
circuit, then check all of the voltage levels at the
offending OBD input, as this error is very likely due to
a wiring problem (see our ‘AN02 - ELM327 Circuit
Construction’ for some typical voltages).
A generic problem has occurred. This is most
often from an invalid signal being detected on the bus
(for example, a pulse that is longer than a valid Break
signal), but usually is from a wiring error. Note that
some vehicles may generate long pulses as part of
their startup process, so you may see this message as
part of a normal vehicle startup while ‘monitoring all.’
The CAN system had difficulty initializing, sending,
or receiving. Often this is simply from not being
connected to a CAN system when you attempt to send
a message, but it may be because you have set the
system to an incorrect protocol, or to a baud rate that
does not match the actual data rate. As with BUS
ERRORs, the CAN ERROR might also be the result of
a wiring problem, so if this is the first time using your
ELM327 circuit, review all of your CAN interface
circuitry before proceeding.
There was a response from the vehicle, but the
information was incorrect or could not be recovered.
There was an error in the line that this points to,
either from an incorrect checksum, or a problem with
the format of the message (the ELM327 still shows
you what it received). There could have been a noise
burst which interfered, possibly a circuit problem, or
perhaps you have the CAN Auto Formatting (CAF) on
and you are looking at a system that is not of the
ISO 15765-4 format. Try resending the command
again – if it was a noise burst, it may be received
correctly the second time.
Elm Electronics – Circuits for the Hobbyist
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Error Messages and Alerts (continued)
There are a number of internal errors that might be
reported as ERR with a two digit code following. These
occur if an internally monitored parameter is found to
be out of limits, or if a module is not responding
correctly. If you witness one of these, contact Elm
Electronics for advice.
One error that is not necessarily a result of an
internal problem is ERR94. This code represents a
‘fatal CAN error’, and may be seen if there are CAN
network issues (some non-CAN vehicles may use pins
6 and 14 of the connector for other functions, and this
may cause problems). If you see an ERR94, it means
that the CAN module was not able to reset itself, and
needed a complete IC reset to do so. You will need to
restore any settings that you had previously made, as
they will have been returned to their default values.
Beginning with v1.3a of this IC, an ERR94 will also
block further automatic searches through the CAN
protocols, if bit 5 of PP 2A is a ‘1’ (it is by default). This
is done because most ERR94s will be as a result of
serious CAN wiring problems. Blocking of the CAN
protocols remains in effect until the next power off and
on, or until an AT FE is sent.
When an OBD output is energized, a check is
always made to ensure that the signal also appears at
the respective input. If there is a problem, and no
active input is detected, the IC turns the output off and
declares that there was a problem with the FeedBack
(FB) of the signal. If this is an initial trial with your
ELM327, this is almost certainly a wiring problem.
Check your wiring before proceeding.
This appears as a warning that the ELM327 is
about to switch to the Low Power (standby) mode of
operation in 2 seconds time. This delay is provided to
allow an external controller enough time to prepare for
the change in state. No inputs or voltages on pins can
stop this action once initiated.
The ELM327 continually monitors the 5V supply to
ensure that it is within acceptable limits. If the voltage
should go below the low limit, a ‘brownout reset’ circuit
is activated, and the IC stops all activity. When the
voltage returns to normal, the ELM327 performs a full
reset, and then prints LV RESET. Note that this type of
reset is exactly the same as an AT Z or MCLR reset
(but it does not print ELM327 v1.4b).
Beginning with v1.3a of this IC, an LV RESET will
also block any automatic searches through the CAN
protocols, if bit 4 of PP 2A is a ‘1’ (it is by default). This
is done because most LV RESETs will be as a result
of overloads due to CAN wiring problems, if they
should start occurring suddenly. Blocking of the CAN
protocols is only done until the next reset (AT Z, WS,
etc.) or until an AT FE is sent.
The IC waited for the period of time that was set
by AT ST, and detected no response from the vehicle.
It may be that the vehicle had no data to offer for that
particular PID, that the mode requested was not
supported, that the vehicle was attending to higher
priority issues, or in the case of the CAN systems, the
filter may have been set so that the response was
ignored, even though one was sent. If you are certain
that there should have been a response, try increasing
the ST time (to be sure that you have allowed enough
time for the ECU to respond), or restoring the CAN
filter to its default setting.
An error was detected in the received CAN data.
This most often occurs if monitoring a CAN bus using
an incorrect baud rate setting, but it may occur if
monitoring and there are messages found that are not
being acknowledged, or that contain bit errors. The
entire message will be displayed as it was received (if
you have filters set, the received message may not
agree with the filter setting). Try a different protocol, or
a different baud rate.
If any OBD operation is interrupted by a received
RS232 character, or by a low level on the RTS pin, the
ELM327 will print the word STOPPED. If you should
see this response, then something that you have done
has interrupted the ELM327. Note that short duration
pulses on pin 15 may cause the STOPPED message
to be displayed, but may not be of sufficient duration to
cause a switch to Low Power operation.
Version 1.3 of the ELM327 first introduced this
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Error Messages and Alerts (continued)
message, but it was only displayed when a protocol
initiation or search was interrupted. As of version 1.4,
the message is printed any time that an OBD task is
The ELM327 has tried all of the available
protocols, and could not detect a compatible one. This
could be because your vehicle uses an unsupported
protocol, or could be as simple as forgetting to turn the
ignition key on. Check all of your connections, and the
ignition, then try the command again.
Outline Diagrams
The diagrams at the right show the two package
styles that the ELM327 is available in.
The first shows our ELM327P product in what is
commonly called a ‘300 mil skinny DIP package’. It is
used for through hole applications.
The ELM327SM package shown at right is also
sometimes referred to as 300 mil, and is often called
an SOIC package. We have chosen to simply refer to
it as an SM (surface mount) package.
The drawings shown here provide the basic
dimensions for these ICs only. Please refer to the
following Microchip Technology Inc. documentation for
more detailed information:
• Microchip Packaging Specification, document name
en012702.pdf (7.5MB). At the www.microchip.com
home page, click on Packaging Specifications, or go
to www.microchip.com/packaging
• PIC18F2480/2580/4480/4580 Data Sheet, document
name 39637d.pdf (8.0MB). At the www.microchip.com
home page, click on Data Sheets, then search for
Note: all dimensions shown are in mm.
All rights reserved. Copyright 2005 to 2010 by Elm Electronics Inc.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
Elm Electronics – Circuits for the Hobbyist
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Absolute Maximum Ratings, 5
Altering Flow Control Messages, 46
Applications, Example, 65-69
AT Commands, 9
AT Command
Descriptions, 11-26
Summary, 9-10
Features, 1
Figure 9, 67
Flow Control Messages, Altering, 46
FMS Standard, 53
Headers, setting them, 37-39
Higher RS232 Baud Rates, 59-60
Battery Voltage, 27
Baud Rates, Using Higher RS232, 59-60
Block Diagram, 1
Bus FMS Standard, 53
Bus Initiation, 33
Initiation, Bus, 33
Inputs, unused, 5
Interface, Microprocessor, 64
Interpreting Trouble Codes, 31
CAN Extended Addresses, Using, 47
CAN Message Formats, 44
CAN Messages and Filtering, 41
Codes, Trouble,
Interpreting, 31
Resetting, 32
Commands, AT
Summary, 9-10
Commands, OBD, 28
Communicating with the the ELM327, 7-8
FMS Standard, 53
Messages, 48-49
Number of responses, 51
Using, 50-53
KeepAlive (Wakeup) Messages, 33
Description and Features, 1
Low Power Operation,
Description, 62-63
Modifications, 70-71
Electrical Characteristics, 6
Error Messages, 72-74
Example Applications
Basic, 65-67
Figure 9, 67
USB, 66-69
Low Power, 70-71
Extended Addresses, CAN, 47
Maximum Ratings, Absolute, 5
Messages and Filtering, CAN, 41
Messages, Error, 72-74
Message Formats, CAN, 44
Message Formats, OBD, 35-36
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Index (continued)
Microprocessor Interfaces, 64
Modifications for Low Power, 70-71
Monitoring the Bus, 40
Multiline Responses, 42-43
Responses, Multiline, 42-43
Restoring Order, 45
RS232 Baud Rates, Using Higher, 59-60
Number of Responses,
J1939, 51
OBDII 30, 61
OBD Commands, 28
OBD Message Formats, 35-36
Order, Restoring, 45
Ordering Information, 5
Outline Diagrams, 74
Overview, 7
Selecting Protocols, 34-35
Setting the Headers, 37-39
Setting Timeouts (AT & ST commands), 61
Specify the Number of Responses, 30, 51, 61
AT Commands, 9-10
Programmable Parameters, 55-58
Talking to the Vehicle, 29-30
Timeouts (AT & ST commands), 61
Trouble Codes,
Interpreting, 31
Resetting, 32
Pin Descriptions, 3-5
Pin 28, resetting Prog Parameters, 55
Power Control,
Description, 62-63
Modifications, 70-71
Programmable Parameters,
general, 54-55
reset with pin 28, 55
Summary, 55-58
types, 55
Protocols, Selecting, 34-35
Quick Guide for Reading Trouble Codes, 32
Wakeup Messages, 33
Unused pins, 5
Using J1939, 50-53
Using CAN Extended Addresses, 47
Using Higher RS232 Baud Rates, 59-60
Voltage, Reading the Battery, 27
Reading the Battery Voltage, 27
Reading Trouble Codes, Quick Guide for, 32
Prog Parameters, 55
Trouble Codes, 32
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