Circuit Description

A closed loop air/fuel metering system is used to provide best possible combination of driveability, fuel economy and emission control. While in closed loop, PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. Scan tester monitors fuel trim values, and will indicate a change made to fuel delivery by long and short term fuel trim values.

Ideal fuel trim values are about 128. If oxygen sensor signal indicates a lean condition, PCM will add fuel, resulting in a fuel trim value greater than 128. If a rich condition is detected, fuel trim values will be less than 128, indicating that PCM is reducing amount of fuel delivered. PCM will set Code P0171 if an excessively lean condition is detected.

Code P0171 will set when the following conditions are present.

• Engine fuel system is operating in closed loop.
• Fuel trim is enabled.
• BARO is greater than 10.2 psi (70 kPa).
• TP sensor angle is less than 70 percent.
• Engine RPM is 500-4000 RPM.
• MAP is 20-90 kPa.
• IAT is between -22°F (-30°C) and 140°F (60°C).
• ECT is between 140°F (60°C) and 239°F (115°C).
• MAF is 5 grams per second and 100 grams per second.
• Vehicle speed is less than 75 MPH.
• EVAP canister purge duty cycle is less than 75 percent.
• Codes P0114, P0115, P0116, P0117, P0118, P0133, P0134, P0153 and P0154 are not set.
• All conditions are met for 10 seconds.
• Average short term fuel trim is 138.
• Average long term fuel trim is 155.