Short-term fuel trim (STFT) is the ECU's live reaction to the O₂ sensor. Long-term (LTFT) is the drift it has learned. When the two disagree, the ECU is trying to cope with something that doesn't fit the learned model.
The four states
- STFT ≈ 0, LTFT ≈ 0: healthy, everything learned.
- STFT positive, LTFT positive: persistent lean — vacuum leak, weak fuel delivery.
- STFT ≈ 0, LTFT positive: ECU has fully compensated for a lean drift — confirmed but not acute.
- STFT positive, LTFT ≈ 0: new lean condition that hasn't been learned yet — intermittent leak, recent failure.
Reading with the 5 gases
Fuel trims confirm or contradict your gas interpretation:
- Gas says lean + trims say lean → genuine lean; investigate.
- Gas says lean + trims say rich → exhaust leak at / before the probe; ECU sees what's in the exhaust, not the cylinder.
- Gas says rich + trims say lean → O₂ sensor lazy or contaminated; trust the gas.
Bank separation
On V-engines, bank 1 vs bank 2 trim differences point directly at bank-specific faults — an injector issue on one head, a vacuum leak on one plenum, a clogged cat on one side.
// rule of three
If STFT and LTFT each exceed ±10% you have a real fault — even if no DTC has set. ECUs log a code once trim authority is exhausted (usually ±25%).