Fuel Trim
Adjustments made by the engine control module to the air-fuel mixture to maintain the stoichiometric ratio.
Fuel trim refers to the dynamic adjustments made by the Engine Control Unit (ECU) to the amount of fuel delivered to the engine's cylinders. The ECU constantly monitors feedback from the oxygen sensors to determine if the combustion mixture is running too rich (excess fuel) or too lean (excess air) and adjusts the fuel injector pulse width to maintain optimal combustion efficiency.
Fuel trim values are expressed as percentages and are divided into Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). A value of 0% indicates that the fuel mixture matches the ECU's pre-programmed baseline. Positive trim values mean the computer is adding fuel to correct a lean condition, while negative values mean the computer is subtracting fuel to correct a rich condition.
If fuel trim percentages drift too far from normal limits (typically beyond +/- 10% to 15%), it indicates an underlying engine performance issue. If trims exceed +/- 25%, the check engine light will illuminate, storing DTCs for lean or rich operation. Diagnosing fuel trim anomalies helps technicians locate issues like vacuum leaks, fuel pump failures, or faulty mass airflow sensors.