Fuel Trim Interpreter

Translate STFT and LTFT numbers into clear lean or rich clues.

Interpret Your Fuel Trims

Use readings from a warm engine in closed loop for the best results.

What Fuel Trims Mean

Short-term fuel trim (STFT) shows rapid ECU adjustments, while long-term fuel trim (LTFT) tracks the running average. Together they reveal if the engine is running lean (positive trim) or rich (negative trim).

  • Normal: Within roughly -5% to +5% total trim.
  • Borderline: 5% to 10% suggests a small bias.
  • High: Over 10% points to a system issue worth chasing.