STFT (Short-Term Fuel Trim)

STFT (Short-Term Fuel Trim) is a real-time correction the engine's DME makes to how much fuel it injects, shown as a percentage. It reacts instantly to oxygen sensor feedback to keep the air-fuel mixture at the stoichiometric ratio (around 14.7:1 for gasoline). A positive STFT means the DME is adding fuel to correct a lean mixture; a negative STFT means it is cutting fuel to correct a rich one.

The DME applies STFT by lengthening or shortening the injector pulse width cycle by cycle. Under normal operating conditions STFT fluctuates rapidly around 0% as the DME makes constant small corrections in response to the oxygen sensor signal.

STFT works in tandem with LTFT (Long-Term Fuel Trim). STFT reacts immediately to oxygen sensor readings on a cycle-by-cycle basis, while LTFT accumulates a learned offset over time. When STFT consistently trends in one direction, the DME shifts that correction into LTFT so that STFT can return closer to zero. If the combined STFT and LTFT correction exceeds the DME's threshold, typically around ±25% on most BMW engines, a fuel trim fault code is set.

On BMW engines with split exhaust manifolds, STFT is tracked independently per bank. Bank 1 and Bank 2 each have their own STFT value, allowing the DME to isolate fueling issues to one side of the engine. A large STFT offset on only one bank typically points to a localized issue such as a vacuum leak at a specific intake runner, a failing injector, or an exhaust leak upstream of that bank's oxygen sensor.

STFT values can be monitored live using ISTA, INPA, or consumer scan tools. When diagnosing fuel trim fault codes, reading STFT alongside LTFT at idle and under load is the most effective first step. The split between the two values and the conditions under which they deviate reveals whether the issue is a steady-state leak, a load-dependent restriction, or a sensor fault.