DTC P0171: Fuel Trim System Lean: Description

WARNING: This page is about a different car, the 2003 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 to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during open and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop the PCM adds oxygen sensor inputs 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 FT values change slower in response to trends in Short Term FT adjustments. Fuel Trim Index is the average of Short and Long Term Fuel Trim and Purge Learn Memory based on engine speed and load. If the PCM detects an excessively lean condition, DTC P0171 sets.