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A Honda civic car with a mileage of about 60 thousand km flooded in 2008. After the car was repaired, it was found that when the ignition was connected, the F - bit relay in the fuse box of the engine room gave a "click" sound, and the guard lamp and the AT indicator on the dashboard were always bright. Fault diagnosis: as the F relay in the fuse box of the engine room is ETCS relay, that is, the electronic throttle control relay. When the relay vibrate, we can hear the vibration of the electronic throttle inside the same frequency. When the relay is unloaded, the sound on the electronic throttle will disappear. But after pulling off the relay, the engine is unable to start. So other relays in the relay box were replaced with the relay, and the malfunction remained. The wire side connector of the electronic throttle is pulled out for trial run, and the failure phenomenon is gone. It is suspected that there is a problem with the electronic throttle. However, after the electronic throttle of a normal model car is removed and replaced by a defective vehicle, the malfunction still exists. Is it a problem with PCM? The car changed the MICU (multiplex controller) because of the accident, and because of the current abnormal phenomenon can not carry on the MICU login, so it can only start from the circuit. According to the analysis of the fault phenomenon, it is considered that the power supply voltage of the system will be abnormal when the power supply system is bad. When the power supply voltage is too low, the relay contacts will not be sufficient to maintain the closure, which makes the system power off. When the system circuit load is disappearing, the power supply voltage of the system increases instantaneously, and the relay is re absorbed again. Such a recurrence may lead to these failures. According to this analysis, the PCM A, B and C wire side splice are checked for the terminals of the 3 connectors respectively. When the terminal B1, B36, C2, C40, C44 are detected, all the breakers are found to be in circuit breakage. Then, following the investigation of the engine wiring harness, it was finally found that the G101 wiring point of the PCM harness was not connected on the cylinder block after the air cleaner housing.