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, ECM 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 ECM is reducing amount of fuel delivered. If a lean condition exists, fuel trim values will be greater than zero percent indicating ECM 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 will set when fuel trim is excessively high on any of the speed/load ranges.

DTC will set if closed loop is active and the following fuel trim limits have been exceeded:

• Idle at 7.7 percent.
• Medium speed/load at 21 percent.
• High speed/load at 21 percent.