DTC P0171: Fuel Trim System - Lean: Description

The 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 operation. During open loop, PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During closed-loop, PCM adds oxygen sensor inputs to calculate 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 Heated Oxygen Sensor (HO2S) voltage signals. The long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. DTC will set if PCM detects an excessively lean condition.