Fuel Trim

Category: automotive

The percentage of adjustment the PCM makes to the fuel injection duration to maintain a perfect air-fuel ratio.

Fuel trim is the "PCM’s correction." Positive trim means the PCM is *adding* extra fuel (usually fixing a vacuum leak). Negative trim means the PCM is *removing* fuel (fixing an over-rich condition). It’s the best way to diagnose hidden engine issues.

Common Examples

• A high positive fuel trim reading at idle strongly suggested a vacuum leak in the intake manifold gasket.
• By monitoring fuel trim via our OBD-II scanner, we were able to pinpoint the failing fuel injector long before it caused a total misfire.