Short Term Fuel Trim

The scan tool range is -11 percent to +20 percent. Short Term FT represents a short-term correction to fuel delivery by the PCM in response to the amount of time the fuel control oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly remained less than 450 mV, indicating a lean air/fuel mixture, Short Term FT will increase into the positive range of more than 0 percent and the PCM will add fuel. If the oxygen sensor voltage stays mainly above the threshold, Short Term FT will decrease to less than 0 percent into the negative range while the PCM reduces fuel delivery to compensate for the indicated rich condition. Under certain conditions such as extended idle and high ambient temperatures, canister purge may cause Short Term FT to read in the negative range during normal operation. The PCM maximum authority to control fuel trim allows a range between -11 percent and +20 percent. Fuel trim values at maximum authority may indicate an excessively rich or lean system.