Fuel Trim

The Engine Control Module controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The Engine Control Module monitors the heated oxygen sensor (HO2S) signal voltage while in closed loop and regulates the fuel delivery by adjusting the pulse width of the injectors based on this signal. The ideal fuel trim values are around 0 percent for both short and long term fuel trim. A positive fuel trim value indicates the Engine Control Module is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the Engine Control Module is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) signal voltage. These changes fine-tune the engine fueling. The long term fuel trim makes rough adjustments to fueling in order to recenter and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The Engine Control Module selects the cells based on the engine speed and engine load. If the Engine Control Module detects an excessively lean or rich condition, the Engine Control Module will set a fuel trim diagnostic trouble code (DTC).