ELM327P datasheet(29/76 Pages) ELM

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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:

01 - show current data

02 - show freeze frame data

03 - show diagnostic trouble codes

04 - clear trouble codes and stored values

05 - test results, oxygen sensors

06 - test results, non-continuously monitored

07 - show ‘pending’ trouble codes

08 - special control mode

09 - request vehicle information

0A - 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:

>AT Z

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.

29 of 76

ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

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