Breakdown of a code

Diagnostic trouble codes are alphanumeric codes that are used to identify a problem that is present on any of the systems that are monitored by the on-board computer (ECM or PCM).

EOBD and OBD II diagnostic codes are composed of five characters; one letter followed by 4 digits, e.g. P0101.

The first character is a letter. It identifies the "main system" where the fault originated. This will be one of the following systems:

Letter

System

B

Body

C

Chassis

P

Powertrain

U

Network (UART)

The second character is a numeric digit. This identifies the type of code.

Code type

Explanation

Generic
(normally P0xxx)

The definition for the code is defined in the EOBD / OBDII standard and will be the same for all manufacturers.

Manufacturer-specific
(normally P1xxx)

Where manufacturers feel that a code is not available within the generic list, they can add their own codes. The definitions for these are set by the manufacturer.

In general, codes that begin with P0 are Generic codes, whereas codes that begin with P1 are manufacturer-specific. Additional code group are however available to allow for expansion of these code lists.

The full breakdown of the code groups is shown below:

Powertrain codes

Body codes

P0xxx - Generic

B0xxx - Generic

P1xxx - Manufacturer-specific

B1xxx - Manufacturer-specific

P2xxx - Generic

B2xxx - Manufacturer-specific

P30xx-P33xx - Manufacturer-specific

B3xxx - Generic

P34xx-P39xx - Generic

 

Chassis codes

Network Communication codes

C0xxx - Generic

U0xxx - Generic

C1xxx - Manufacturer-specific

U1xxx - Manufacturer-specific

C2xxx - Manufacturer-specific

U2xxx - Manufacturer-specific

C3xxx - Generic

U3xxx - Generic

The third digit defines the specific system or sub-system within the car where the problem is located:

Third digit

System or sub-system

1

Fuel and Air Metering

2

Fuel and Air Metering (injector circuit malfunction only)

3

Ignition System or Misfire

4

Auxiliary Emission Control System

5

Vehicle Speed Control and Idle Control System

6

Computer Output Circuits

7

Transmission

8

Transmission

It should be noted that manufacturers are not required to follow the generic numbering system above for their manufacturer-specific codes (many do appear to follow these though).


The fourth and fifth digits identify the section of the system causing the fault. There is no detailed list of the components indicated by these digits, as each system or sub-system will have many different components.