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Fault phenomenon: a czecheta auto, when the engine tachometer pointer goes away, the oil warning lamp sometimes alarm, the automatic transmission accelerates the failure phenomenon, and the above failure phenomenon always appears and disappears when it is lost. Inspection and analysis: connect VAGl551 to detect engine and automatic transmission control system, and do not find fault code storage. The engine control system reads the relevant data flow, shows the engine speed of 840r/min, and enters the automatic transmission brother control system to read the related data flow, which shows that the engine speed is only 90r/min. The engine speed signal is obtained by the engine rotational speed inductor (G28), then the signal is transmitted to the engine control unit. The engine control unit is output in 2 ways, and the engine tachometer on the dashboard is input all the way. The automatic transmission control unit is entered all the way. The automatic transmission control unit controls the timing of shift by means of engine speed signal, throttle opening signal and lucky speed signal. Considering that the engine system is completely normal, the engine speed sensing brocade (G28) and the related line failure possibility can be excluded because once the sensing brocade and line problems will affect the normal operation of the engine. For the oil warning lamp, it is considered that the system is related to the engine speed signal. On the bumper, the oil warning system will check the oil pressure when the engine is idle and the engine speed is 2000 r/min. When the oil warning system relay does not receive the engine speed signal, the raging warning lamp will call the alarm. The automatic transmission and tachometer fail at the same time, but the engine system is normal, indicating that the fault may only appear in the output part or the signal line of the engine control unit. As the engine control unit has 2 paths to output the engine speed signal, and the possibility of the 2 line faults at the same time is very small, so the fault occurs on the engine control unit. Then the engine control unit of the normal same type car was replaced by the replacement test. The failure phenomenon disappeared, thus indicating that the fault was caused by the engine control unit. Troubleshooting: replace engine control unit and match it according to technical requirement.