Specifications

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Interpreting Trouble Codes

Likely the most common use that the ELM323 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’)

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, then, subtract 128 (or 80

hex). When the result is less than 128, simply read the

number of stored codes directly.

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 SAE

document J1979 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:

>03

A response to this could be:

43 01 33 00 00 00 00

The ‘43’ in the above response simply indicates

that this is a response to a mode 03 request. The other

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:

Powertrain Codes - SAE defined

“ “ - manufacturer defined

“ “ - SAE defined

“ “ - jointly defined

If the first hex digit received is this,

Replace it with these two characters

Chassis Codes - SAE defined

“ “ - reserved for future

Body Codes - SAE defined

Network Codes - SAE defined

“ “ - reserved for future

“ “ - 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’). As for

further examples, if the response had been D016, the

code would be interpreted as U1016, while a 1131

would be P1131.

More than one ECU module can respond to

requests such as this, so be prepared to possibly

receive several lines of responses. To determine

which ECU is reporting each line would require turning

the headers on with the AT H1 command.

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