Circuit Description

PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on the signal voltage. A change made to fuel delivery is indicated by the long and short term fuel trim values.

Short term fuel trim values change rapidly in response to HO2S signal voltages. These changes fine tune engine fueling. Long term fuel trim values changes in response to trends in short term fuel trim. Long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. Long term and short term fuel trim can be monitored by use of scan tool.

Ideal fuel trim value is about zero percent. A positive fuel trim indicates that PCM is adding fuel to compensate for a lean condition. A negative fuel trim indicates that PCM is reducing amount of fuel to compensate for a rich condition. DTC will set if PCM detects an excessively rich or lean condition.