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:

• Long and short term fuel trim is 30 percent or more for 5 seconds.
• Short term fuel trim is 20 percent or more for 45 seconds.