Lean vs rich: fuel trims that actually help
Fuel trims are one of the fastest truth sources you have. Used correctly, they stop parts-darting and point to air, fuel delivery, or sensor plausibility.
Start here: what numbers matter
- Short-term fuel trim (STFT): the ECU's moment-to-moment correction.
- Long-term fuel trim (LTFT): the learned correction over time.
- Idle vs 2,500 rpm: compare both. The difference tells you if the problem is airflow/leak related or fuel delivery related.
Lean pattern (positive trims)
- High at idle, improves with rpm: vacuum/PCV leak, intake gasket leak.
- High everywhere: low fuel pressure/volume, MAF under-reporting, exhaust leak upstream of O2.
- One bank only: bank-specific air leak, injector imbalance, upstream O2 bias (verify).
Rich pattern (negative trims)
- Rich at idle: leaking injector, fuel pressure regulator fault, purge valve stuck open (EVAP).
- Rich under load: MAF over-reporting, restricted intake, fuel pressure too high, incorrect sensor scaling.
- One bank only: injector on that bank, intake runner issue, sensor bias (verify).
Confirmatory tests (fast and cheap)
- Smoke test for intake/PCV leaks (especially if idle trims are high).
- MAF plausibility: compare to MAP/load, and check for contamination or aftermarket filters/oil.
- Fuel pressure at idle and under load - pressure can look fine at idle and collapse on demand.
- Purge valve pinch-off (where safe) to see if trims stabilise (common rich-at-idle cause).
Related code pages
P0171
System too lean - how to split leaks vs fuel delivery.
Open code ->P0172
System too rich - common causes and quick tests.
Open code ->
Trust note: Trims are a direction, not a verdict. Confirm with a test that changes the condition (smoke test, pressure test, pinch-off) before buying parts.