Fuel trim (STFT / LTFT)
Definition
Fuel trim is the percentage by which the ECU adjusts the base injection quantity to keep the lambda sensor reading at stoichiometric. Short-term fuel trim (STFT) reacts within seconds to the sensor's switching; long-term fuel trim (LTFT) is the learned average stored in memory, by RPM and load range, so that STFT can return to oscillating around zero. Positive values mean the ECU is adding fuel to correct a lean condition (vacuum leaks, weak pump, dirty injectors, under-reading MAF, exhaust leaks before the sensor); negative values mean it is removing fuel to correct a rich condition (leaking injectors, high fuel pressure, stuck purge valve, over-reading MAF). Most systems flag a code when total trim exceeds roughly plus or minus 25%; a trim high at idle but normal under load points to a vacuum leak, while a trim high under load points to fuel delivery. Codes: P0171/P0174 (system too lean, bank 1/2), P0172/P0175 (too rich), P2187-P2190 (lean/rich at idle), P2195-P2198 (lambda signal stuck lean/rich).