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Understanding OBD Fault Codes / DTCs (Diagnostic Trouble Codes)

There is a collection of trouble codes called standard or generic that are used by all manufacturers.

Any of the following names for these codes, or a variation of those names, may be used:

  • DTC (Data/Diagnostic Trouble Code)

  • OBD (On-Board Diagnostic)

  • Fault Codes

  • Trouble Codes

Diagnostic equipment may read and interpret this list of codes. They follow a format as detailed below.

Explaining Their Format

Below is an image of an example fault code, P0419, each colour represents a specific meaning as detailed below.

old fault code example

First Letter (Orange)

The family of DTC is indicated by the first letter, which is shown in orange in the image above.

  • P: Powertrain

  • C: Chassis

  • B: Body

  • U: User network

First Digit (Green)

The first digit, as shown in green in the image above, indicates whether or not the fault code is generic or manufacture.

  • 0: Generic fault

  • 1: Manufacturer fault

Manufacturers sometimes add their own fault codes if they require it because the list of generic OBD codes is not always adequate.

Last Three Digits (Blue)

The last three digits, as shown in blue in the image above, indicate the type of fault that has occurred.

For codes beginning with P, sub-families are defined using the first digit of the last three digits ("4" in the example image above).

It may also be a hexadecimal number (from 0-9 and A-F).

  • 0, 1 & 2: Air/fuel mixture

  • 3: Ignition system

  • 4: Auxiliary emissions

  • 5: Engine idling

  • 6: Onboard computer and ancillary outputs

  • 7, 8 & 9: Transmission (gearbox)

  • A, B & C: Hybrid propulsion


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