DTC P0172: Fuel Trim System Rich: Description

WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. 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 differently during open loop 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 the short and long term fuel trim (fuel delivery adjustments). If the oxygen sensors indicate a lean condition, the fuel trim values will be above zero percent. If the oxygen sensors indicate a rich condition, the fuel trim values will be below zero percent. The short term fuel trim values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. The long term fuel trim makes coarse adjustments 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 the PCM detects an excessively rich condition, Diagnostic Trouble Code (DTC) P0172 sets.