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. DTC P0171 will set when air fuel ratio is stable after engine is in closed loop operation and fuel trim indicates considerably rich.

When DTC sets, MIL will illuminate. PCM will record operating conditions at time of failure and store information in freeze frame buffer.