Circuit Description

A closed loop air/fuel metering system is used to provide the best combination of driveability, fuel economy and emission control. When in closed loop mode, PCM monitors HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values, which can be monitored with a scan tool. Ideal fuel trim values are around zero percent. If HO2S signals are indicating a lean condition, the PCM will add fuel, resulting in fuel trim value greater than zero percent. If a rich condition is detected, fuel trim values will be less than zero percent, indicating that PCM is reducing the amount of fuel delivered. An excessively lean or rich condition detected will set a DTC.