Fuel Trim

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📖 YOUCANIC Automotive Glossary

Fuel trim is the ECU’s adjustment to injection pulse width expressed as a percentage. STFT (Short-Term Fuel Trim) is the immediate correction based on O2 sensor feedback. LTFT (Long-Term Fuel Trim) is the stored averaged correction. Positive values mean adding fuel (system lean); negative means subtracting fuel (system rich). Normal combined trim should stay within plus or minus 10%.

Monitoring fuel trims is the most effective technique for mixture diagnosis. High positive LTFT at idle that normalizes at highway speed indicates a vacuum leak. High positive LTFT worsening under load indicates fuel delivery problems. When trims exceed ~25%, the ECU sets P0171/P0174 (lean) or P0172/P0175 (rich). Compare banks: if both deviate equally, the cause affects both (fuel pump, MAF); if only one bank, the cause is bank-specific.

Synonyms:
STFT, LTFT, Short Term Fuel Trim, Long Term Fuel Trim
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