Circuit Description

WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

The DTC P0171 Fuel Trim System Lean diagnostic monitors Heated Oxygen Sensor (HO2S) indications of air/fuel mixture. 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. PCM monitors HO2S signal voltage and when in closed loop adjusts fuel delivery based on HO2S signal voltage. Changes in fuel delivery will be indicated by long term and short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around zero percent. PCM will add fuel when heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above zero percent. PCM will reduce the amount of fuel delivered when a rich condition is indicated by HO2S. Fuel trim values below 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. See FUEL TRIM SYSTEM OPERATION table.

FUEL TRIM SYSTEM OPERATION

HO₂ S Indication	Short Term FT Action	Long Term FT Action	Short Term FT Response
Lean Condition	Quick Increase of 1% Or More	Slow Increase Of 1% Or More	Return To Zero Percent
Desired Condition	Stays Near Zero Percent	Stays At Learned +/- Value	Stays Near Zero Percent
Rich Condition	Quick Decrease Of -1% Or Less	Slow Decrease Of -1% Or Less	Return To Zero Percent