A Shanghai Volkswagen Santana 2000GSi car with a mileage of about 130 thousand km. The car appears difficult to start the cold car failure phenomenon. Fault diagnosis: using fault detector to detect, no trouble code storage. Check the ignition system, intake pipe, engine timing and associated tie line, no abnormalities. Clean the carbon deposit of throttle and intake pipe and match after trial run. According to the maintenance experience, if the above checks are normal, it is likely that the starting of cold trucks is caused by the excessive concentration of mixed gas. The engine can start the engine smoothly when the engine is sprayed to the throttle, which indicates that the reason for the hard start of the cold car is that the concentration of the mixture is too thin. After the engine starts the engine, the fuel pressure is measured to 280 kPa ~ 300 kPa, normal; the engine is extinguished, and the test of the pressure of the fuel line is normal. After the engine cold machine reads the engine data flow, it is found that the coolant temperature is 95 C, and the actual coolant temperature is only about 20 degrees C, thus it is inferred that the coolant temperature sensor has trouble. The coolant temperature sensor of the car has 4 terminals, of which terminal 1 and terminal 3 provide the coolant temperature signal for the engine control unit, and terminal 2 and terminal 4 to provide the coolant temperature signal for the instrument. Disconnect the ignition switch, remove the coolant temperature sensor wire connector, and use the multimeter to measure the resistance between the temperature sensor terminal 1 and terminal 3 of the coolant, which is about 200 omega (around 2 Omega normal at 20 C), which indicates that the coolant temperature sensor is damaged. Troubleshooting: replacement of coolant temperature fault analysis: the vehicle fault is caused by drift of coolant temperature sensor resistance. When cold starting, the resistance of the coolant temperature sensor is about 200 Omega, the engine control unit mistakenly thinks that the coolant temperature is 95 C, and the control reduces the injection pulse width, so that the mixture concentration is too thin, so that the engine is difficult to start. The cause of the fault is not complex, for the experienced maintenance personnel, the diagnosis may be directly from the coolant temperature sensor, but because of no fault code hints, the possible reasons for the failure are more, sometimes it is unavoidable to take a curve.