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

DTC will set if fuel trim index is greater than 165 (32 percent) for 4 seconds and:

• ECT between 140-239°F (60-115°C).
• MAP pressure is greater than 20 kPa on M/T or 28 kPa on A/T.
• Engine speed at 550-3600 on A/T or 750-3600 on M/T.
• BARO greater than 72 kPa.
• IAT reading between -13°F and 239°F (-25°C and 115°C).
• Fuel level greater than 9.8 percent.
• Vehicle speed less than 82 MPH.
• No engine sensor, VSS, EVAP canister purge, front O2S, idle speed, PCM internal, misfire, injector or P0125 DTC have been set.