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, control module monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. 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 about 128. If oxygen sensor signal indicates a lean condition, control module 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 control module is reducing amount of fuel delivered. Control module will set code if an excessively rich condition is detected.

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