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A motor vehicle with a mileage of about 24 thousand km, equipped with a 2.3L engine, matching the 5 gear hand / self transmission, the FAW Mazda 6 passenger car in 2008. User response: the engine fault warning light of the vehicle is lit, so it is difficult to start and can not be accelerated. After further enquiry, the user knew that the car had been dragging its bottom at low speed, and the engine fault warning light was lit. The user found the local repair shop for inspection and found that there were no other injuries except for the slight damage of the engine belt. To this end, the maintenance personnel go to the rescue. After arriving at the scene, the maintenance personnel use the fault diagnosis instrument to detect the engine control system. The equipment indicates that there is a fault in the ignition timing. Because the site can not be solved, the car will be dragged back to the maintenance station. Fault diagnosis: after the connection, the connection fault diagnosis instrument WDS detects the engine control system, and the device shows the fault code "-CMP (camshaft position signal) too early". According to the maintenance data, in the normal working temperature conditions, when the engine speed is lower than 4000r/min, if the actual valve timing is more than 17 degrees ahead of the target, the generator control unit will record the fault code. According to the fault code analysis, the causes of the failure include the oil control valve (OCV) failure, the plunger stuck in the timing advance position, the variable valve timing actuator stuck in advance position, the valve timing error (the instantaneous change of the speed may cause the tooth belt hopping) and the PCM fault. Using fault diagnosis instrument to observe the dynamic data flow of engine control system is as follows. Engine speed: 560r/min (normal value: 700r/min)