Long Term FT

The scan tool range is -23 to +16 percent. The long term fuel trim is derived from the short term fuel trim value. This represents a long term correction of the fuel delivery. A value of 0 percent indicates that fuel delivery requires no compensation in order to maintain the PCM commanded air/fuel ratio. A negative value significantly below 0 percent indicates that the fuel system is rich and fuel delivery is being reduced, decreased injector pulse width. A positive value significantly more than 0 percent indicates that a lean condition exists and the PCM is compensating by adding fuel, increased injector pulse width. Because long term fuel trim tends to follow short term fuel trim, a value in the negative range due to the canister purge at idle should not be considered unusual. The PCMs maximum authority to control long term fuel trim allows a range between -23 percent and +16 percent. Fuel trim values at maximum authority may indicate an excessively rich or lean system.