Circuit Description

To provide the best possible combination of driveability, fuel economy and emission control, a closed loop air/fuel metering system is used. While in closed loop, ECM monitors heated oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made in fuel delivery will be indicated by 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 change 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. Short and long term fuel trim can be monitored with a scan tool. Ideal fuel trim values are around zero percent. A positive fuel trim value indicates ECM is adding fuel to compensate for lean condition. A negative fuel trim value indicates ECM is reducing amount of fuel to compensate for rich condition.

If an excessively rich or lean condition is detected, ECM will set a DTC. Long term (additive and multiplicative) fuel trim diagnostic parameter is a calculated value from several of long term speed loan learn cells which are selected based on speed and load.

Conditions for setting DTC:

• ECM detects fuel trim values greater than 550 milliseconds for additive fuel trim or greater than 23 percent for multiplicative fuel trim.