The mileage is about 50 thousand km, the vehicle type is C6, matching the 01J transmission, the Audi A6L 2.8 passenger car equipped with BDX engine in 2010. Users reflect: the car's water temperature is high at low speed. Inspection and analysis: maintenance personnel road test found that the car does not turn on the air conditioning on the urban road, occasionally there will be high water temperature. After the air conditioner is turned on, it runs about 30 km and the coolant temperature reaches 130 degrees. Using VAS6150B to detect the engine control unit, it is found that there is a hint that the cooling fan actuator 1 is short circuiting the grounding. The actuator is tested, and the fan is working normally. The power supply voltage of each control unit is normal. According to the principle of detection when the failure occurs, the air conditioner is opened to speed up the engine temperature. When the temperature rises to 100, the fan is suddenly stopped running. At this time, the fan power supply voltage is normal, but there is no output current, which indicates that there is a virtual connection phenomenon in the fan circuit. According to the circuit diagram, check the contact point of radiator fan fuse S42 (60A). It is found that the color change has occurred and the connection has loosened. The analysis shows that the S42 connection is loose and the resistance increases after the fan works for a period of time because of the temperature rise. When the engine temperature reaches the need of the cooling of the fan, the resistance has increased to the state that the current can not be provided. Troubleshooting: re - fastening the contact after grinding, troubleshooting.