Fuel Trim

📖 YOUCANIC Automotive Glossary

Fuel Trim (Short-Term and Long-Term)

Fuel trim is the ECU’s real-time adjustment to the fuel injection pulse width, expressed as a percentage, that compensates for deviations from the ideal 14.7:1 air-fuel ratio (stoichiometric ratio for gasoline engines). Short-Term Fuel Trim (STFT) is the immediate, moment-to-moment correction the ECU makes based on the upstream oxygen sensor’s feedback — it reacts in real time, fluctuating rapidly between positive and negative values as the ECU constantly fine-tunes the mixture. Long-Term Fuel Trim (LTFT) is the learned, averaged correction the ECU applies over time to compensate for persistent deviations — it represents the ECU’s memory of how much the fuel delivery needs to differ from the base fuel map to maintain the correct mixture. A positive fuel trim value (e.g., +10%) means the ECU is adding fuel because the system is running lean; a negative value (e.g., -10%) means the ECU is subtracting fuel because the system is running rich.

Monitoring fuel trim values through your OBD2 scanner’s live data function is the most effective technique for diagnosing air-fuel mixture problems. Normal fuel trim values should stay within approximately -10% to +10% combined (STFT + LTFT) at both idle and cruising RPMs. If total fuel trim at idle exceeds +15-20%, the system is running lean — common causes include vacuum leaks, weak fuel pump, clogged fuel injectors, faulty MAF sensor reading low, or an intake manifold gasket leak. If fuel trim is significantly negative (-15% or more), the system is running rich — common causes include leaking fuel injectors, a faulty MAF sensor reading high, stuck-open purge valve, or high fuel pressure. The pattern of fuel trim across different operating conditions reveals the root cause: high positive LTFT at idle that normalizes at highway speed points to a vacuum leak (the leaked air is a smaller percentage of total airflow at higher RPM). High positive LTFT that gets worse at higher RPM and load points to a fuel delivery problem (pump or filter can’t keep up with demand). When fuel trim exceeds approximately ±25%, the ECU sets a DTC — P0171/P0174 for lean conditions or P0172/P0175 for rich conditions. Always check both Bank 1 and Bank 2 fuel trims: if both banks show the same deviation, the cause is something that affects both banks equally (fuel pump, fuel pressure, MAF sensor); if only one bank is affected, the cause is specific to that bank (vacuum leak on one side, clogged injector on one bank).