Description of On-Board Diagnostics

The PCM performs an on-board diagnostic check of the EGR system. The diagnostic system uses the electronic EGR transducer for the system tests.

The diagnostic check activates only during selected engine/driving conditions. When the conditions are met, the PCM energizes the transducer solenoid to disable the EGR. The PCM checks for a change in the heated oxygen sensor signal.

If the air-fuel mixture goes lean, the PCM will attempt to enrichen the mixture. The PCM registers a Diagnostic Trouble Code (DTC) if the EGR system is not operating correctly.
- After registering a DTC, the PCM turns on the malfunction indicator -- indicating the need for service -- after 2 consecutive trips.

There are 2 types of failures sensed by the PCM. The first is a short or open in the electrical solenoid circuit. The second is a mechanical failure or loss of vacuum.

If a problem is indicated by the MIL and a DTC for the EGR system is set, check for proper operation of the EGR system.
- Use the System Test, EGR Gas Flow Test.

If the EGR system tests properly, check the system using a scan tool and the appropriate Powertrain Control System/Computers and Control Systems diagnostic procedures.