Circuit Description

To provide best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. During closed loop, PCM monitors heated oxygen sensor signal voltage and adjusts fuel delivery accordingly. A change made to fuel delivery is indicated by long and short term fuel trim values and can be monitored using a scan tool. Ideal fuel trim values are around zero percent.

If a rich condition exists, fuel trim values will be less than zero percent indicating PCM is reducing amount of fuel delivered. If a lean condition exists, fuel trim values will be more than zero percent indicating PCM is increasing amount of fuel delivered. A fuel trim DTC can set if exhaust emissions exceed preset levels because of a lean or rich condition.

When DTC sets, MIL will illuminate. PCM will record operating conditions at time of failure and store information in FREEZE FRAME/FAILURE RECORDS data buffer. PCM will enter Fail-Safe Function. EGR and EVAP purge control will be stopped.