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.