DTC P0171: Fuel Trim System Lean: Description

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.

Powertrain Control Module (PCM) controls air/fuel metering system in order to provide best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during open and closed loop. 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 more than zero percent. If oxygen sensors indicate a rich condition, fuel trim values will be less than zero percent. Short Term Fuel Trim (STFT) values change rapidly in response to Heated Oxygen Sensor (HO2S) voltage signals. Long Term Fuel Trim (LTFT) makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. DTC sets when PCM detects an excessively lean condition.