On-Board Diagnostics (OBD)

On-Board Diagnostics (OBD) - North American Specification Vehicles Only
The ECM monitors performance of the engine for misfires, catalyst efficiency, exhaust leaks and evaporative control loss. If a fault occurs, the ECM stores the relevant fault code and warns the driver of component failure by illuminating the Malfunction Indicator Light in the instrument pack.

On vehicles fitted with automatic gearbox, the ECM combines with the Electronic Automatic Transmission (EAT) ECU to provide the OBD strategy.

Conditions
If the OBD function of the ECM flags a fault during its operation, it falls into one of the following categories:
^ mm minimum value of the signal exceeded.
^ max = maximum value of the signal exceeded.
^ signal signal not present.
^ plaus = an implausible condition has been diagnosed.

Function
All of the ECM's internal diagnostic fault paths are monitored by the OBD system. Specific faults have their own numeric code relating to certain sensors or actuators etc. These specific faults fall into two types, error codes (E xxx) or cycle codes (Z xxx). E codes represent instantaneous faults and Z codes relate to codes generated after completion of a drive cycle.

If an emission relevant fault occurs on a drive cycle, the ECM stores a temporary fault code, if the fault does not occur on subsequent drive cycles the fault code stays as a temporary fault code. If the fault recurs on subsequent drive cycles the ECM stores the fault code as a permanent code, and depending on which component has failed the ECM will illuminate the MIL.