Short Term Fuel Trim (STFT)
Immediate, temporary corrections made by the engine computer to the fuel delivery based on real-time feedback from the oxygen sensors.
Short Term Fuel Trim (STFT) represents the immediate, second-by-second adjustments made by the Engine Control Unit (ECU) to the fuel injector open duration. Driven by feedback from the pre-catalytic oxygen sensors, STFT reacts instantly to changes in exhaust oxygen levels, fluctuating constantly to keep the air-fuel ratio as close to stoichiometric as possible.
STFT values are highly volatile. When an oxygen sensor detects excess oxygen (lean), STFT instantly rises to a positive percentage to add more fuel. Conversely, when the sensor detects unburned fuel (rich), STFT drops to a negative percentage to restrict fuel. Once the ignition is turned off, the STFT values are reset and not stored in the computer's long-term memory.
While normal STFT oscillates rapidly, a persistent positive or negative value indicates that the ECU is constantly fighting to correct a fuel imbalance. If the STFT remains elevated (for example, at +20%) for an extended period, the ECU will eventually transfer these corrections into the Long Term Fuel Trim (LTFT) database to establish a new fuel baseline.