Fuel trim is the correction the engine computer applies to the base fuel calculation to keep the air-fuel mixture correct, based on oxygen sensor feedback. Short-term fuel trim reacts instantly, while long-term fuel trim learns and stores the correction. Positive numbers mean the computer is adding fuel because the engine is running lean; negative numbers mean it is removing fuel because the engine is running rich. Fuel trims are the single most useful scan tool reading for diagnosing lean and rich codes.
Short-term vs long-term fuel trim
| Reading | What it does |
|---|---|
| Short-term fuel trim (STFT) | Changes many times a second as the oxygen sensor switches; it should move around zero |
| Long-term fuel trim (LTFT) | Learns the average correction over time and stores it, so short-term trim can stay near zero |
| Total fuel trim | STFT plus LTFT: the overall correction the computer is making |
What is a normal fuel trim?
Total trim within roughly plus or minus 10 percent is generally considered normal. Larger corrections mean something is adding unmeasured air or changing fuel delivery. Most engine computers set a lean or rich code when trim stays near its limit, which is often around 25 percent; see P0171, P0174, P0172 and P0175.
Using fuel trims to diagnose
- High positive trim at idle that improves at 2,500 RPM: a vacuum leak, because a fixed leak is a larger share of airflow at idle.
- High positive trim at all speeds: low fuel pressure, a clogged fuel filter or a MAF sensor under-reading airflow.
- High positive trim under load only: a weak fuel pump or restricted filter.
- Negative trim (rich): a leaking injector, high fuel pressure (see fuel pressure regulator) or a sensor reporting the engine as cold (see coolant temperature sensor).
- One bank different from the other: a problem on that bank only, such as an intake gasket, injector or exhaust leak.
How to read fuel trims
- Warm the engine fully so the system is in closed loop.
- Record STFT and LTFT for each bank at idle, then at a steady 2,500 RPM.
- Check freeze-frame data from the code: it shows engine speed, load and trims when the fault set.
- After a repair, clear long-term trims (clearing codes usually does this) and confirm they settle near zero.
Fuel trim and oxygen sensors
Trims are only as good as the upstream oxygen sensor. A slow or biased sensor makes trims misleading, and an exhaust leak ahead of it makes the engine look lean; see O2 sensor. Lean running over long periods can burn valves and pistons; if a compression test shows damage, compare replacements on our engines page.
More guides: is your fuel pump big enough?.
Trouble codes: P2187 System Too Lean at Idle · P2188 System Too Rich at Idle · P2270 O2 Sensor Stuck Lean B1S2 · P2271 O2 Sensor Stuck Rich B1S2 · all OBD2 codes.
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