Beijing modern Sonata car with a mileage of about 15 thousand km. The owner reflected that when the vehicle was running on the hot car, the water temperature instructions sometimes reached the red line, which was normal when the car was cold, and the speedometer sometimes did not work. Fault diagnosis: the maintenance technician check after the car, the hot car to the normal working temperature, use the diagnostic instrument to observe the temperature of the water in the sensor data flow, and find the water temperature rise to more than 100 C. At this time the cooling fan has long run (high speed gear), and the temperature difference of the water pipe is obvious with the hand. It can be found that the thermostat is broken (open at 82 degrees C). Open 93 degrees C) and replace the thermostat. When the hot car reaches normal working temperature, the water temperature in the sensor data stream is normal, and the water temperature indicator is normal at this time. Again, after 10min, it was found that the water temperature meter indicated that the water temperature was normal and that the temperature difference between the upper and lower pipes was not obvious, and the water temperature was normal. From the point of failure to the circuit problem, the resistance of the meter to the water temperature sensor is measured by multimeter. The resistance value is 65~67 ohms, which belongs to the normal range. Then the instrument was replaced and the road test was repeated. Less than half an hour, the water temperature indicator reached the red line again, and the failure could not be ruled out. The factory carefully inspected the line and measured the resistance of the instrument. It was found that the resistance was very unstable and changed above 36 ohms. Find the tie point (after the indoor junction box), found that the bolt has been unable to tighten. After treatment, the old instrument is replaced, the road test is installed, the water temperature indicator has been normal, the speedometer work is normal, and the trouble is completely eliminated. Failure summary: the fault of the vehicle is that the water temperature is high but the actual water temperature is normal. At the beginning, it is not understood, after the analysis, this is because in the cold car, because the water temperature sensor resistance is larger, higher than the instrument's iron resistance, the instrument does not string from the water temperature meter to the water temperature sensor, so the water temperature is normal; when the water temperature rises, the sensor resistance is lower than the instrument's iron resistance, the instrument inside the instrument. The part of the circuit will be attached by the water temperature sensor, so the water temperature indication is also high, causing a wrong indication. When the meter is poor, the speedometer can not work properly, which is why the speedometer is sometimes absent.