DTC P0172: Fuel Trim System - Rich: Description

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 differently during open loop and closed-loop. During open loop, PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During closed-loop, oxygen sensor inputs are added and used by PCM to calculate the short and long term fuel trim (fuel delivery adjustments). If oxygen sensors indicate a lean condition, fuel trim values will be greater than zero percent. If oxygen sensors indicate a rich condition, fuel trim values will be less than zero percent. The short term fuel trim values change rapidly in response to HO2S voltage signals. The long term fuel trim makes coarse adjustments in order to maintain air/fuel ratio of 14.7:1. The 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 PCM detects an excessively rich condition, this DTC will set. If PCM detects excessive vapor, then a pass is logged.