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 (ISO 15031-5) standard, 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 ELM329 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, and you may wish to
reset it by sending:
>AT Z
If you have just powered up the ELM329 circuit,
you do not need to do this (as it happens automatically
with every power on). You will see the interface LEDs
flash, and then the IC should respond with ‘ELM329
v2.1’, followed by a prompt character. At this point, you
may choose a protocol that the ELM329 should
connect with, but it is usually best 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
ELM329 for communicating with a vehicle, and often
you do not even need to do that - most times you can
simply jump ahead to the next step. You do not need
to know anything about the protocol that your vehicle
uses - the ELM329 will determine that for you.
At the prompt, issue the mode 01 PID 00
command:
>01 00
The ELM329 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
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ELM329
ELM329DSC
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