- Positive trim generally adds fuel; negative trim generally removes it.
- Compare banks and operating states.
- Confirm closed-loop status and sensor validity.
- Use airflow, pressure, oxygen response, load, and fuel-delivery evidence together.
STFT and LTFT
Short-term trim is the controller’s faster correction. Long-term trim is learned correction stored across an operating region. Their displayed sum can help describe total correction at that moment, but strategies, limits, and reset behavior vary by calibration.
Pattern changes matter
A positive correction strongest at idle that improves with RPM can fit an unmetered-air pattern. A correction that grows under load can fit delivery or airflow-measurement limits. A bank-to-bank split can focus attention on bank-specific leaks, injectors, exhaust leaks, or sensing. These are test directions, not conclusions.
Prerequisites before interpretation
Verify the engine is warm when the test calls for it, closed loop is active, the correct fuel is present, battery voltage is stable, and no obvious mechanical or exhaust fault invalidates the feedback. Review purge command, fuel pressure if available, misfire counters, and oxygen or air-fuel sensor behavior.
A repeatable workflow
Record warm idle, a stable elevated-RPM no-load point, and a safe steady-load segment. Compare bank behavior and repeat after any controlled change. Smoke testing, pressure measurement, injector balance, and scope testing may be needed before a cause can be confirmed.
What fuel-trim number is bad?
There is no single universal threshold for every vehicle and condition. Exact service limits, combined correction, bank spread, and repeatable operating patterns matter.
Do positive trims prove a vacuum leak?
No. Unmetered air is one possibility; fuel delivery, airflow measurement, exhaust leaks, purge flow, and mechanical conditions can create similar corrections.
This is general educational material designed to improve questions and evidence capture. Definitions, thresholds, enabling conditions, wiring, service steps, and safety requirements can differ by vehicle application. Use current official manufacturer information and qualified judgment for the actual repair.