Circuit Description

The DTC P0171 Fuel Trim System Lean diagnostic monitors the heated oxygen sensor indications of the air/fuel mixture. In order to provide the best possible combination of driveability, fuel economy, and emission control, the Powertrain Control Module (PCM) uses a Closed Loop air/fuel metering system. The PCM monitors the Heated Oxygen Sensor (HO2S) signal voltage and when in Closed Loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around zero percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are greater than zero percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values less than zero percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.