What Is Fuel Trim? STFT and LTFT Explained

Fuel trim is a percentage that shows how much the ECU adds or removes fuel to hit the ideal air/fuel mixture. There are two kinds: short-term (STFT), the instant adjustment, and long-term (LTFT), the learned average.

Normal values

Generally ±10% is considered normal. Small plus/minus values are expected. If values stay above +10%, the engine is running lean (adding fuel); below -10%, it's running rich.

High positive trim (lean)

• Intake/vacuum leak
• Dirty MAF sensor
• Weak fuel pressure or clogged injector

High negative trim (rich)

• High fuel pressure or leaking injector
• Faulty MAF/MAP sensor
• Dirty air filter

Watching fuel trims is one of the best ways to catch a problem before codes like P0171 or P0172 appear. AutoMulti shows these values live and flags anomalies.