Long Term Fuel Trim

The scan tool displays -10 percent to 10 percent. Long Term FT is derived from the Short Term FT value and represents a long-term correction of fuel delivery. A value of 0 percent indicates that fuel delivery requires no compensation 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 FT tends to follow Short Term FT; a value in the negative range due to canister purge at idle should not be considered unusual. The PCMs maximum authority to control Long Term FT allows a range between -10 percent and +10 percent. Fuel trim values at or near maximum authority may indicate an excessively rich or lean system.