A Shanghai Volkswagen Santana 2000 limousine with a mileage of over 200 thousand km. Users reflect: the car idling out of control, cold car 1050r/min, hot car 1500r/min, and drift uncertain. The owner reflected that after the battery was damaged, a new generator was replaced and the battery was charged. Fault diagnosis: first, use the decoder to read the fault code, for the normal system without fault code, indicating that the electronic control system is basically normal, then transfer the doubt to the mechanical part, remove the idle air inlet hose, in the state of the car, the idle speed of the intake valve is stopped by the hand, the idle speed can be reduced with the block area increased. It's very low, even out of fire. It is proved that there is no air leakage in the inlet. The doubt came back to the electronic control section again. Using decoder to read data flow, it is found that the duty cycle of idle valve varies between 4%-36% and is unstable. The waveform of idle valve is observed by an oscilloscope, and the resistance of idle valve is 17fl, normal. But the troubleshooting should be from simple to complex, so a new idle valve test has been replaced, but the failure phenomenon has not changed. The idle speed of the control unit controls the damage of the power block. After the replacement of a new engine control unit, the trouble is eliminated.