DTC P0172: Fuel Trim System Rich: Description

WARNING: This page is about a different car, the 2002 Pontiac Sunfire, 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, and 2002 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 in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled during open and closed loop operation. 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, fuel delivery adjustments. Short term fuel trim values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain air/fuel ratio of 14.7:1. The fuel trim index is the average of short and long term fuel trim 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, this DTC will set. If PCM detects excessive vapor, then a pass is logged.