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The Volkswagen city golf car with a mileage of about 103 thousand km. The car coolant temperature warning lamp alarm. Fault check: according to the principle analysis of the temperature alarm system of the coolant of the car, there are two cases of this fault: (1) the fault of the circuit control system; and the failure of the cooling cycle system. First check the circuit system, use the multimeter to measure the control line without circuit breakage and short circuit, and then replace the coolant temperature sensor and instrument assembly, but the failure is still, obviously malfunction in the cooling cycle system. The engine is accelerated to 2500r/min, and there is no fault in half an hour. Then drove out of the factory for trial run. When the car was running 22km, the coolant temperature lamp suddenly reported to the police. Then, parking inspection, found that the coolant is lower than the water level down line, and when the radiator lid is opened, the coolant goes back up, then the coolant is added to the appropriate position, and the car is opened back to the service station. It is found that the pump head has a certain loose, replaced or the original sample, and the radiator and water flanges are replaced after the replacement. Finally, the cylinder head is removed and the cylinder pad is checked. It is found that there is a path of water rust in the four cylinder port and the cylinder liner, which causes the coolant to slowly sink into the cylinder and even the circulation system can not operate normally when the engine is running for a long time. Finally, the cooling fluid temperature is raised and the cooling liquid temperature lamp alarm. Troubleshooting: reduction of all spare parts, replacement of cylinder mats, driving after 40km above all normal reaction, troubleshooting.