Circuit Description
WARNING: This page is about a different car, the 1995 Oldsmobile Ninety-Eight and 1995 Buick Park Avenue. However, it is still accessible from the selected car via links, so may be relevant.
A closed loop air/fuel metering system is used to provide best possible combination of driveability, fuel economy and emission control. While in closed loop, PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. Scan tester monitors fuel trim values, and will indicate a change made to fuel delivery by long and short term fuel trim values.
Ideal fuel trim values are about 128. If oxygen sensor signal indicates a lean condition, PCM will add fuel, resulting in a fuel trim value greater than 128. If a rich condition is detected, fuel trim values will be less than 128, indicating that PCM is reducing amount of fuel delivered. PCM will set Code P0171 if an excessively lean condition is detected.