06/21/2026
MCB Autotech – Tuning Tech Sundays
Short Term Fuel Trim (STFT)
Short Term Fuel Trim (STFT) represents the ECU’s immediate correction. Using feedback from the oxygen or air-fuel ratio sensors, the ECU makes rapid adjustments to injector pulse width to maintain the commanded air-fuel ratio. These are the fast, real-time corrections happening every second as operating conditions change.
When STFT is positive, the ECU is adding fuel because it detects a lean condition. When STFT is negative, the ECU is removing fuel because it detects a rich condition. Think of STFT as the ECU’s instant reaction to what it is seeing right now.
Long Term Fuel Trim (LTFT)
Long Term Fuel Trim (LTFT) is the ECU’s learned correction over time. When the ECU detects a repeated pattern in STFT adjustments, it begins storing those corrections as learned values. Instead of constantly making the same short-term adjustment, it applies a baseline correction to improve efficiency and reduce unnecessary compensation.
LTFT essentially represents the ECU’s memory. It reflects long-term trends caused by changes such as sensor aging, airflow deviations, fuel delivery changes, or calibration inaccuracies.
Fuel trims reveal how much correction the ECU is making to maintain proper fueling. Small corrections usually indicate accurate airflow and fuel calculations, while large positive or negative trims suggest something has changed. Positive trims mean the ECU is adding fuel (lean condition), while negative trims mean it’s removing fuel (rich condition), often pointing to issues like vacuum leaks, MAF scaling errors, injector issues, or fuel delivery problems.
From a tuning perspective, fuel trims are one of the clearest windows into calibration accuracy. They help us understand not just what the engine is doing, but how much correction is required to achieve the desired result.
At MCB Autotech, tuning goes beyond making power. We analyze how the ECU learns, adapts, and compensates because understanding those corrections leads to better drivability, reliability, and performance.