Short Term FT

The scan tool range is -11% to 20%. The short term fuel trim represents a short-term correction to the 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 fuel trim increases into the positive range above 0% and the PCM adds fuel. If the oxygen sensor voltage stays mainly above the threshold, short term fuel trim decreases below 0% into the negative range while the PCM reduces fuel delivery in order to compensate for the indicated rich condition. Under certain conditions, such as extended idle and high ambient temperatures, canister purge may cause Short Term Fuel Trim to read in the negative range during normal operation. The PCM's maximum authority to control fuel trim allows a range between -11% and +20%. Fuel trim values at maximum authority may indicate an excessively rich or lean system.