Fuel Trim Live Data: STFT and LTFT
Fuel trim data is the most powerful diagnostic stream available through OBD-II, allowing you to see precisely how the ECU is compensating for deviations from the ideal 14.7:1 stoichiometric air-fuel ratio. Short Term Fuel Trim (STFT) is the ECU's immediate, real-time correction based on the upstream oxygen sensor signal — values range from -25% to +25%. A positive STFT means the ECU is adding fuel (the mixture is running lean); a negative STFT means it is removing fuel (running rich). Long Term Fuel Trim (LTFT) is the ECU's learned correction accumulated over multiple drive cycles — it represents the base-line deviation from zero. Healthy fuel trim: both STFT and LTFT should be within ±5% at idle and ±8% under load. LTFT values outside ±10% indicate a systemic problem. High positive LTFT at idle (+15% or more): common causes are intake/vacuum leaks (unmetered air bypassing the MAF sensor), a weak fuel pump (insufficient pressure), or clogged injectors. High positive LTFT that normalizes at higher RPM: almost certainly an idle-area vacuum leak, because vacuum leak effect is greatest at idle. High negative LTFT: indicates too much fuel — a leaking injector (stuck open), a faulty fuel pressure regulator holding excessive pressure, or an evaporative emissions leak allowing fuel vapor into the intake.