The mileage is about 122 thousand km, and the engine and transmission models are B4204S and MPS6 Volvo S40 cars respectively. In the course of the car, the water temperature meter falls down. Fault diagnosis: when the car is about 5min, the pointer of the water temperature meter suddenly falls to the lowest level, the fan runs at high speed, and the sideslip indicator lights flicker. The possible causes of fault analysis: (1) water temperature sensor damage; (2) open circuit or short circuit of water temperature sensor line; (3) central electronic control unit to instrument CAN line fault; (4) engine control unit to central electronic control unit CAN line fault; 5 contact plug has corrosion oxidation phenomenon. First connect the VIDA diagnostic instrument to read the related fault code, only one: "ECM-0118 engine coolant temperature sensor, signal too high, B420S3 0" according to the fault code, at least can exclude the network CAN line, because in the communication control unit if one side of the fault will produce the related fault code, water temperature The sensor is also rarely bad. In order to judge its good or bad, it can only take the resistance. The resistance value is 37k Omega under 25 degrees. It is normal, and the water temperature sensor fault is eliminated. So the circuit diagram can only be opened. It can be seen from Figure 1 that the water temperature sensor is connected directly with the engine control unit. First, it takes two lines to take off, and has no circuit breakage. The C5 signal voltage is 4.7V and the signal is normal. At this time, the wire harness plug can only be handled, including the central central electronic control unit plug, after processing. The failure of the test is not reproduced, always feeling the failure is not so simple, so do not let the customer lift the car, in place to watch the car idle running watch water temperature meter, water temperature pointer suddenly fell to the lowest, the engine accompanied with jitter flicker flicker flicker. Where is the problem? At this time, lost in the confusion, can only be back to the original point of thought, from the failure phenomenon can only be caused by the lack of water temperature signal, that is, the current failure phenomenon and the extraction of water temperature plug is the same, so we want to remove the water temperature sensor plug remeasurement resistance, water temperature sensor is negative temperature coefficient, and That is to say, the higher the temperature, the lower the resistance, the greater the resistance and the lower the temperature. In the process of pulling the plug, it is found that the water temperature plug can actually turn. Troubleshooting: replace the water temperature sensor and solve the problem of trial run. Fault summary: in view of this kind of fault phenomenon must be carefully checked, when the failure occurs in the process of investigation must be careful thinking, can not be too random, otherwise it will be half the effort, so the idea must be clear.