Fuel Trim

The control unit calculates fuel from air flow, then fine-tunes it with feedback from the oxygen sensor. Short-term fuel trim reacts within fractions of a second; long-term fuel trim learns a lasting correction. Positive trim means the control unit is adding fuel because it sees a lean mixture; negative trim means it is removing fuel. When the long-term correction reaches its limit, it sets a lean (P0171/P0174) or rich (P0172/P0175) code.

Short-term fuel trim reacts within fractions of a second; long-term fuel trim learns lasting corrections. Values are given in percent, positive meaning more fuel is being added.

Trims are shown as percentages on a scan tool. Comparing them at idle and at higher speed helps tell vacuum leaks (worse at idle) from fuel delivery or air flow measurement problems. The oxygen sensor reports whether the mixture is rich or lean, and the control unit adjusts injection time. Trims are reported per cylinder bank on V engines.

Fuel trims are one of the most useful diagnostic values. They show which way and how far the mixture is wrong before a misfire or code appears. Trims that sit at a high positive value point to a lean condition such as a vacuum leak; large negative values point to a rich condition. Live data shows this before a fault code appears.