Fuel Trims (STFT / LTFT)

The ECU's percentage corrections to fueling based on oxygen sensor feedback, short-term and long-term.

Fuel trims are the percentage adjustments the ECU applies to its fuel calculation to hit stoich in closed loop. Short-Term Fuel Trim (STFT) reacts instantly to the narrowband oxygen sensor and bounces around zero; Long-Term Fuel Trim (LTFT) is the learned running average the ECU stores to correct persistent error. They matter because they are your scorecard for the base fuel calibration: trims near zero across the map mean your VE or MAF table is accurate, while large positive trims mean it is running lean (adding fuel) and large negative means rich. Use them, not just the wideband, to dial in volumetric efficiency. In HP Tuners VCM Scanner, log STFT and LTFT as channels and watch them by load and RPM. When tuning the VE table you typically aim to flatten trims to single digits. Note trims only operate in closed loop; under PE/WOT the ECU stops trimming, so confirm WOT fueling with a wideband.