Intake air leaks
Faulty front heated oxygen sensor
Faulty fuel injectors
Incorrect fuel pressure
Lack of fuel
Faulty Mass Air Flow (MAF) sensor
Incorrect Positive Crankcase Ventilation (PCV) hose connection
Engine Light ON (or Service Engine Soon Warning Light)
This fault code is stored in the engine control module when the ECM is no longer capable of controlling the air-fuel ratio. For correct operation of the catalytic converter, the ECM is required to keep the air-fuel ratio (AFR) at approximately lambda 1, this equates to an AFR of 14.7:1 by mass. This is achieved by trimming the fuel mixture to the required amount to cater for slight anomalies during normal operating conditions. However, when a fault causes the fuel trim to exceed a predetermined level the malfunction indicator light is illuminated, and a limited operating strategy is employed. The pre-catalyst exhaust gas oxygen sensor is the primary component used for calculating the fuel trim once the system enters closed-loop control. The fuel trim data can be displayed as a percentage, either positive or negative.
Under this condition, the fuel trim data will display a positive value. The ECM will increase the injection duration to counteract the excessively lean mixture. This fault can be stored due to an air leak in the induction system, the exhaust system in close proximity to the exhaust gas oxygen sensor, low fuel pressure, a faulty fuel injector or a mechanical defect affecting the engine.
The fuel trim is monitored under various loads and conditions. If a load or condition is included in the fault code this can be an advantage during diagnosis. Normally a code set during idle or light load is related to an air leak. And a code set during high or part load can be a fuelling issue or an erroneous sensor signal.