Testing fuel trim

Fuel trim is a correction made to fuel injector on time or pulse width based on readings from the air fuel and oxygen sensors. Short term fuel trim (STFT) is a "momentary" correction that is made to keep the air fuel sensor centered on the ecu's target value. (+%) values mean that fuel is added to the base pulse width and (-%) values mean fuel is subtracted. If STFT moves away from 0 long enough, then Long term fuel trim (LTFT) makes a correction to try and keep STFT centered on 0, and stores that value in memory. Add STFT to LTFT to get total fuel trim.

Ideally, fuel trims should stay near zero, as the factory designed. However, any changes (such as wear or problems) can cause trims to change. So really, just drive the car and monitor what the trim values are under different rpm and load conditions.

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Great reply above.....usually they will max at about 20%( 30%trucks) but that will trigger a lean of rich code. 5% either way is very common, and 10% is fine. If more than that , something is usually going on.