Explanation of an EOBD / OBDII fault code (DTC)

What is a Diagnostic Trouble Code (DTC)?

Diagnostic trouble codes (or fault codes) are codes that are stored by the on-board computer diagnostic system. These are stored in response to a problem found in the car by the system. These codes are stored, for example, when a sensor in the car reports a reading that is outside a pre-defined range, e.g. fuel mixture too rich.

The codes identify a particular problem area and are intended to provide the technician with a guide as to where a fault might be occurring within a car. The codes should be used in conjunction with the vehicle's service manual to ascertain which systems, circuits or components should be tested to fully diagnose the fault.

Reliance on codes alone

Parts or components should not be replaced with reference to only a DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.

For example, if a DTC reports a sensor fault, replacement of the sensor is unlikely to resolve the underlying problem. The fault is most likely to be caused by the systems that the sensor is monitoring, but might even be caused by the wiring to the sensor itself.

DTCs may also be triggered by faults earlier down the line. For example, a dirty MAF sensor might be causing the car to overcompensate in its fuel-trim adjustments. As a result, oxygen sensors are likely to report fuel mixture problems.

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 / OBD-II 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 groups are however available to allow for expansion of these code lists.

The full breakdown of the code groups is shown below:

Powertrain codes

P0xxx - Generic

P1xxx - Manufacturer-specific

P2xxx - Generic

P30xx-P33xx - Manufacturer-specific

P34xx-P39xx - Generic

Chassis codes

C0xxx - Generic

C1xxx - Manufacturer-specific

C2xxx - Manufacturer-specific

C3xxx - Generic

Body codes

B0xxx - Generic

B1xxx - Manufacturer-specific

B2xxx - Manufacturer-specific

B3xxx - Generic

Network Communication codes

U0xxx - Generic

U1xxx - Manufacturer-specific

U2xxx - Manufacturer-specific

U3xxx - Generic

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

Third character	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 characters 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.