DTC P0172: Fuel Trim System Rich: Description

WARNING: This page is about a different car, the 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) controls the air/fuel metering system to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled during Open and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate Short and Long Term Fuel Trim (FT), fuel delivery adjustments. Short Term FT values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. Long Term FT values change slower in response to trends in Short Term FT adjustments. The fuel trim index is the average of Short and Long Term FT and purge learn memory based on engine speed and load. Fuel trim diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the Evaporative Emission (EVAP) canister is causing a rich condition. If the PCM detects an excessively rich condition, DTC P0172 sets.