A FAW Volkswagen Jetta GIX car with a mileage of over 280 thousand km in 2003. Users reflect that the temperature of the coolant of the car is high. Inspection analysis: the maintenance personnel check found that when the cooling fluid temperature indication in the vehicle's combination instrument reaches 130 degrees C, the coolant temperature read through the fault diagnosis instrument is only 102 C, thus judging the problem out on the instrument. Further reading of other data indicates that the power supply voltage of the engine control unit at this time is only 10.3 V, and is constantly changing and abnormal. When checking the power supply, it is found that the iron wire connecting the side of the battery with the body is seriously oxidized. When the current flows through the wire, the contact resistance will decrease the voltage of the electric appliance on the vehicle, and of course, it includes the instrument control unit.