Long Term Fuel Trim

The scan tool range is -23 percent to +16 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 PCM's maximum authority to control long term FT allows a range between -23 percent and +16 percent. Fuel trim values at or near maximum authority may indicate an excessively rich or lean system.