A South East V3 rhombus car with a mileage of about 70 thousand km in 2009. The user reflects the engine fault light on the car. Inspection and analysis: the maintenance staff detected the engine control unit and found that all cylinders had different numbers of misfire records. Because of the multi cylinder fire, we first check all the systems outside the engine. Check oil pressure, fuel injector and spark plug jump fire is normal. After removing the fault code, observe the idle speed of the engine. It was found that the engine's idle speed was steady, unlike the fact that many cylinders were not working properly. Observe idle air intake and fuel injection correction, basically normal. In view of the fact that the real cause of the fault can not be seen from the surface phenomenon, the output signal waveform of the crankshaft position sensor is observed with an oscilloscope. It is observed that the pulse interval of the signal is inconsistent, which is not like the signal corresponding to the smooth running of idle speed. Apart from checking the outer cover of the timing belt, a bolt was found to fall off, and the signal wheel of the crankshaft position sensor was struck and deformed. Troubleshooting: replacing the signal wheel of the crankshaft position sensor and troubleshooting.