Short-term fuel trim (STFT) responds quickly to oxygen sensor feedback; long-term trim (LTFT) is a slower-learned adjustment. When they disagree, it tells you something about the nature of the fault.
Typical Patterns
Both Negative (Rich Compensation)
STFT negative (up to -20%) and LTFT negative (e.g., -10%): indicates a steady rich condition — possibly leaking injector, faulty fuel pressure regulator, or dirty MAF over-reporting airflow.
Both Positive (Lean Compensation)
STFT positive (+10-20%) and LTFT positive (+5-15%): indicates a steady lean — vacuum leak, weak fuel pump, clogged injector, or MAF under-reporting.
Discrepancy: STFT Fluctuates Around Zero, LTFT Negative
This pattern suggests a richer-than-stoichiometric engine during warm-up that the ECU has compensated for long-term, but short-term corrections are now minimal. Could be an enrichment that no longer occurs (e.g., stuck-open purge valve that's now closed) or a sensor drift issue.
Discrepancy: STFT Fluctuates Wildly, LTFT Near Zero
The ECU is making constant short-term corrections but hasn't learned a long-term offset. This points to an intermittent issue: an occasional vacuum leak, an injector that sticks, or a sensor that drifts only under certain conditions.
Combine with 5-Gas Data
Fuel trims alone don't tell you the cause. Combine with gas readings:
- Rich trims + high CO + low O₂ → fueling issue (too much fuel)
- Lean trims + low CO + high O₂ → vacuum leak or lean condition
- High HC with either trim pattern → ignition misfire
Example
Idle readings: CO 1.8%, O₂ 0.2%, STFT -15%, LTFT -12%. Both trims negative, mixture rich. Likely cause: fuel delivery too high (weak return line, leaking injector, overpressure).
Conclusion
Look at the relationship between STFT and LTFT, not just their absolute values. Patterns tell you whether a fault is steady, intermittent, or historical. Use gas measurements to classify the mixture (rich/lean) and then narrow down the subsystem.