Code P0172, Fuel Trim Rich: 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 P0172 if an excessively rich condition is detected.

1. Visually/physically checking components/systems which may cause a rich condition may determine cause of Code P0172.
2. MAF sensor which causes PCM to read a higher then actual airflow can cause a rich condition, and Code P0172 to set.
3. Silicon contamination of Heated Oxygen Sensor HO2S can cause a false rich indication that will be sensed by PCM. PCM will then reduce amount of fuel delivered, possibly causing severe driveability symptoms.