The Volkswagen Passat B5 car with a mileage of about 130 thousand km in 2006. The water temperature meter often indicates that the water temperature is too high. After entering the plant, the water temperature indicator of the dashboard reaches the maximum after running for half an hour (after summer). This fault belongs to the typical water temperature meter indicating too high. First of all, it is necessary to determine whether the water temperature meter or the fault of the water temperature sensor on the engine. Then the infrared temperature measuring instrument is used to check the temperature of the water. It is found that the temperature of the thermostat has reached 94, and the actual temperature of the engine at this time should be about 96~100, indicating that the water temperature sensor on the engine and the water temperature indication on the panel are no problem. When the radiator is checked, the electronic fan is not turned to determine whether it is the problem of the water temperature sensor on the water tank. Again, the temperature of the water tank and the temperature sensor is measured again with a thermometer. It is found that the temperature value and its change are also normal. The temperature of the water tank decreases from the upper to the downward, only the drop is not much (the electronic fan is not transported. " Turn, the effect of heat dissipation is not so good. The temperature of the water temperature sensor is also above 95 C. The above check indicates that the large circulation of the cooling system is no problem. Now the core problem is why the electronic fan is not controlled by the temperature of the engine, and the water temperature reaches the temperature that should be operated by the electronic fan, but it does not turn. Then open the air conditioning switch and find that the electronic fan operates normally, indicating that the electrical fan and the electronic fan itself have no problems. The problem should be in the temperature control system. So the water temperature sensor on the water tank is replaced, and the malfunction still exists, and the electronic fan does not turn. Then the resistance simulator is used to simulate the resistance of the water temperature sensor, and the ignition switch is switched on. When the resistance is adjusted to 160 Omega, the low speed electronic fan starts smoothly. When the resistance is adjusted to 150 Omega, the high speed electronic fan starts smoothly. At the same time, the resistance of the water temperature sensor at different temperatures was also measured. It was found that the resistance of the actual test is 180 ohms at 97 C. This indicates that the sensor has errors with the computer, which comes from the internal resistance of the computer. In order to save the cost, we do not change the computer, but through calculation, the water temperature sensor parallel 1 1400 Omega resistors, so that the water temperature at 97 degrees C, the resistance after the parallel is exactly 160 Omega, the computer correctly control the electronic fan start operation, the fault is successfully eliminated. Application analysis of engine exhaust system: exhaust manifold of engine mainly includes exhaust manifold, three element catalyst, oxygen sensor, exhaust pipe and exhaust muffler and so on. The exhaust manifold is used to guide the high temperature exhaust gas generated by each cylinder of the engine, and the exhaust gas is discharged into the atmosphere through a three way catalytic converter, an exhaust pipe and an exhaust muffler. When the exhaust gas flows through the three element catalyst, the redox reaction occurs inside, and the CO, HC and NOx in the exhaust gas are converted into harmless CO2, H2O and N29, and a certain amount of heat is released. The oxygen sensor is used to monitor the oxygen content in the exhaust gas. Through the monitoring of the oxygen content in the exhaust gas, the pre oxygen sensor can reflect the concentration of the mixed gas. The post oxygen sensor can reflect the performance of the three element catalyst. The engine exhaust system and temperature related faults mainly include the failure or blockage of the three element catalyst, and the oxygen sensor can not work normally. In addition, the temperature contrast of the exhaust manifold can reflect the bad work of the individual cylinder. 1. the temperature detection and analysis of the exhaust manifold will vary greatly because of the influence of the different working conditions, different ambient temperatures and the cooling strength of the engine. Therefore, the temperature detection and analysis of the exhaust manifold is mainly compared to the temperature of each cylinder, thus judging the bad work of the individual cylinder, at this time. The engine usually shows idling instability, jitter or intermittent flameout. In the vehicle equipped with OBD II system, the engine fault lights are generally lighted and the fault codes are set. The infrared temperature sensor can be used to scan the position of each cylinder exhaust manifold near the exhaust valve, and the exhaust temperature of each cylinder can be obtained. It should be noted that after the engine works for a long time, the heat transfer of exhaust manifolds will occur, so it is difficult to determine the difference of exhaust temperature between cylinders. Therefore, the temperature detection of the exhaust manifold is as much as possible when the engine is just starting, and the engine temperature is not too high, and the best results can be obtained. If the exhaust temperature of a cylinder shows a lot lower than the other cylinder exhaust temperature (100 degrees centigrade or more), it indicates that the cylinder does not work well, does not burn or does not burn at all. It may be caused by the weak spark plug or no fire, injector blockage or cylinder sealing. Step test the ignition system components, fuel supply system and cylinder pressure of the cylinder. If a cylinder exhaust temperature shows a lot higher than the other cylinder exhaust temperature, it indicates that the mixture concentration of the cylinder is high, and some fuel continues to burn during the exhaust process. At this time, the injector and control circuit of the cylinder should be inspecting emphatically, and the intake valve lift of the cylinder is also sufficient. 2. three element catalytic converter temperature detection and analysis of three element catalyst in normal working state, due to the oxidation reaction will produce a lot of heat, so the temperature difference can be used to determine the performance of the three element catalyst. Temperature detection and analysis can be used to judge the performance of three way catalytic converter. When the engine is tested, the engine is preheated to the normal working temperature, the engine speed is kept around 2500r/min, the vehicle is lifted, and the temperature of the inlet and outlet of the three element catalyst is measured by the infrared thermometer. Note: as far as possible, close to three yuan catalytic converter (50mm). Because of the three element catalyst in normal working state, due to the oxidation reaction will produce a lot of heat, so the temperature of the three element catalyst outlet should be at least 10% ~ 15% at the inlet temperature, and the temperature of the most normal three yuan catalyst outlet is higher than the inlet temperature 20% to 25%. If the vehicle is equipped with a three way catalytic converter before the main three way catalytic converter, the outlet temperature of the main catalytic converter should be higher than that of the inlet temperature of 15% to 20%. If the value of the outlet temperature is lower than the above value, it is indicated that the three element catalytic converter is not working normally, has failed, and needs to be replaced (three yuan catalyst is basically a fault free device, but it is too long in use, such as more than 200 thousand km, the engine uses leaded gasoline, and the long term temperature is too high); if the outlet temperature is low At the inlet temperature or lower than the inlet temperature, the three element catalytic converter is blocked or seriously blocked (the main body of the three catalyst is a honeycomb ceramic carrier, which is easily broken and blocked in the case of impact and strong vibration). If the value of the exit temperature exceeds the above value, the high concentration of CO and HC is contained in the exhaust gas. Further inspection of the engine itself is required. It is necessary to explain that for vehicles equipped with OBD II system, when the three element catalytic converter fails, the engine fault self diagnosis system will light the engine fault light and set the corresponding fault code. 3. oxygen sensor temperature detection and analysis temperature analysis method can also detect oxygen sensor system. After the engine starts, the oxygen sensor must reach the voltage signal at 315 degrees C. If a car takes a long time to enter the closed loop state or does not enter the closed loop control mode, the temperature of the oxygen sensor can be monitored to judge whether the heating line is the problem or the control line.