Long Term FT

The scan tool range is -23% to 16%. Long Term FT is derived from the short term fuel trim value and represents a long-term correction of fuel delivery. A value of 0% indicates that fuel delivery requires no compensation to maintain the PCM commanded air/fuel ratio. A negative value significantly below 0% indicates that the fuel system is rich and fuel delivery is being reduced (decreased injector pulse width). A positive value significantly more than 0% 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 canister purge at idle should not be considered unusual. The PCM's maximum authority to control long term fuel trim allows a range between -23% and +16%. Fuel trim values at or near maximum authority may indicate an excessively rich or lean system.