Short-Term and Long-Term Fuel Trims: What They Tell You About Your Engine

What Are Fuel Trims?

The ECU constantly measures the oxygen content in the exhaust (via O2 and wideband sensors) and adjusts the fuel injector pulse width to maintain the target air/fuel ratio. Fuel trims represent this correction as a percentage.

Short-Term Fuel Trim (STFT): Real-time correction. Changes rapidly in response to current sensor data. Normal range: -5% to +5%.

Long-Term Fuel Trim (LTFT): Learned correction accumulated over time. Represents the average error the ECU has been compensating for. Normal range: -10% to +10%.

Reading the Numbers

Positive fuel trim (e.g., +8%): ECU is adding fuel — the engine is running lean. It detected less fuel than expected and is compensating by injecting more.

Negative fuel trim (e.g., -7%): ECU is removing fuel — the engine is running rich. More fuel than expected is present.

Common Causes of High Positive Trims (Lean)

• Vacuum leak (unmetered air entering after the MAF)
• Dirty or failing MAF sensor
• Fuel pressure too low (weak fuel pump)
• Clogged fuel injectors
• Exhaust leak before the O2 sensor (pulls in outside air, fools sensor)

Common Causes of High Negative Trims (Rich)

• Leaking fuel injector (leaks fuel even when closed)
• High fuel pressure (regulator stuck)
• Faulty coolant temp sensor (ECU thinks engine is still cold, runs rich)
• EVAP purge valve stuck open (vapor canister dumps fuel vapor)

Fuel Trims and Modifications

After installing a cold air intake, LTFT may shift positive temporarily while the ECU relearns. After 2–3 drive cycles, trims should return to normal as the MAF calibration adapts.

If trims remain high positive after an intake install, the intake may be disrupting MAF airflow. A tune recalibrating the MAF transfer function will resolve this.

How to Monitor

Any OBD-II scanner with live data shows STFT and LTFT for Bank 1 (driver side on HEMI) and Bank 2 (passenger side). Monitor both banks — a discrepancy between banks points to a bank-specific issue (one O2 sensor, one bank's injectors, etc.).