Long Term Fuel Trim

Scan Tool Range -23 percent to 16 percent. Long Term Fuel Trim is derived from the Short Term Fuel Trim value and represents a long-term correction of fuel delivery. A value of 0 percent indicates that fuel delivery requires no compensation in order to maintain the PCM commanded air-to-fuel ratio. A negative value significantly below 0 percent indicates that the fuel system is rich and fuel delivery is being reduced. This is indicated by a 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. This is indicated by an 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 percent and +16 percent. Fuel trim values at or near maximum authority may indicate an excessively rich or lean system.