System Description

To provide best possible combination of driveability, fuel economy. and emission control. a Closed Loop air/fuel metering system is used. While in Closed Loop, powertrain control module (PCM)/engine control module (ECM) monitors oxygen sensor (O2S1) signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by long and short term fuel trim values which can be monitored with scan tool. Ideal fuel trim values are around 128 (0%). If O2S1 signal is indicating a lean condition, PCM ECM will add fuel resulting in fuel trim values above 128 (0% to 100%). If a rich condition is detected, fuel trim values will be below 128 (0% to -100%). indicating that PCM/ECM is reducing amount of fuel delivered. If exhaust emissions reach an excessive level due to a lean or rich condition. a fuel trim Diagnostic Trouble Code (DTC) is set.