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Home >> Chevrolet >> 2009 >> Impala Police >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls And Fuel - 3.9L - Introduction >> Description and Operation >> Fuel System Description >> Short Term Fuel Trim

Short Term Fuel Trim

The short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the engine control module (ECM) is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition.

When the ECM determines that the short term fuel trim is out of the operating range, the following DTCs will set:

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