Fuel Trim

SHORT TERM FUEL TRIM
The short term fuel trim values change rapidly in response to the 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 PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition.

When the PCM determines that the short term fuel trim is out of the operating range, the following Diagnostic Trouble Codes (DTCs) will set:
^ DTC P0171 Bank 1 Too Lean
^ DTC P0172 Bank 1 Too Rich

LONG TERM FUEL TRIM
The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). As the engine operating conditions change, the PCM will switch from cell to cell. The PCM uses the value stored in the active long term fuel trim cell to calculate the injector pulse width.

The PCM also monitors the short term fuel trim while in any given cell. If the short term fuel trim is far enough from 0 percent, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, the short term fuel trim should change back toward 0 percent. If the mixture is still not correct, the short term fuel trim will continue to have a large deviation from the ideal 0 percent. In this case, the long term fuel trim value will continue to change until reaching the value's limit. If the mixture is not corrected by the short term and long term fuel trim at their limits, the following DTCs will set:
^ DTC P0171 Bank 1 Too Lean
^ DTC P0172 Bank 1 Too Rich

Under the conditions of power enrichment, the PCM sets the short term fuel trim to 0 percent until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the power enrichment condition.