Code P0171, Fuel Trim Lean: Notes

A closed loop air/fuel metering system is used to provide the 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. The Tech 1 monitors fuel trim values, and will indicate a change made to fuel delivery by the long and short term fuel trim values.

Ideal fuel trim values are approximately 128. If oxygen sensor signal indicates a lean condition, the 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 the PCM is reducing the amount of fuel delivered. PCM will set Code P0171 if an excessively lean condition is detected.

1. Visually/physically checking components/systems which may cause a lean condition may determine cause of Code P0171.
2. MAF sensor which causes PCM to read a lower than actual airflow can cause a lean condition, and Code P0171 to set.
3. Check for faults in EVAP purge system that may cause Code P0171 to set.