Long-term fuel trim
Long-term fuel trim: STFT is short-term closed-loop lambda correction while LTFT represents a slower learned trend. The page also includes a worked example and measurement sequence.
Technical frame
Long-term fuel trim: STFT is short-term closed-loop lambda correction while LTFT represents a slower learned trend. The page also includes a worked example and measurement sequence.
The technical values here expose the standard, protocol or physical relationship directly; model-specific service values are linked through the matching model/variant dossier.
The goal is not only to define the term but to let the reader calculate and interpret what the data means in a scan, scope or physical test.
Concrete technical facts
- STFT is short-term closed-loop lambda correction while LTFT represents a slower learned trend.
- Positive fuel trim means the ECU is adding fuel to the base calculation; negative trim means removing fuel.
- Comparing STFT+LTFT at idle and elevated rpm/load helps separate vacuum leaks from fuel-supply problems.
- Fuel trim is meaningful in the appropriate closed-loop/monitor conditions; interpretation changes in open loop, WOT or warm-up.
- Bank 1 vs Bank 2 differences can point to bank-specific air/fuel or sensor faults.
- Lambda-sensor bias or exhaust leaks can drive trim even without the assumed fuel-system fault.
Worked example
- Example: STFT +8% and LTFT +12% gives about +20% combined adding-fuel tendency. If +20% at idle falls to +3% at 2500 rpm, a vacuum-leak pattern becomes plausible.
Measurement and verification sequence
- Confirm closed-loop state.
- Record bank-specific STFT/LTFT at idle and load.
- Correlate with MAF/MAP/lambda and fuel pressure.
- Physically test intake and exhaust for leaks.
Fault-separation logic
- Is a valid command present and are power/ground/network healthy?
- Does feedback follow the command?
- Does an independent physical measurement confirm the output?
- Is the fault limited to a specific temperature/load/speed condition?
- Does the result remain stable when the original condition is repeated after repair?
Technical sources
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Model-specific real technical data · Measurement references · Technical diagnostic atlas