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 scan tool. Ideal fuel trim values are around zero percent. Fuel trim values less than zero percent indicate a rich condition, while values greater than zero percent indicate a lean condition. DTC P0171 will set when:

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