So what is fuel trim? Fuel trim is rebalancing the fuel mixture for reduced emissions. It is a number that represents the computer’s learned amount of total fuel need. That number is derived from the time the fuel injectors are pulsed on and off. The higher the fuel-trim number, the more fuel is being commanded, the more injector pulse-on time. The lower the number, the less fuel is commanded, the less the injector is pulsed on. The fuel-trim command is a result of what the oxygen sensor tells the computer. You don’t want to see short-term fuel trim over 10 on your scan tool. Fuel trim starts at 0 and can go positive (more fuel) or negative (less fuel).