A red flag CA7220E car with a mileage of over 280 thousand km in 1998. The car was repaired because of unstable engine operation and high idle speed. Fault diagnosis: after the first test, it is found that the engine is unstable, the idle speed fluctuates, and the speed is sometimes 3000r/min. In fact, there is no idle speed. Only when the throttle sensor plug is pulled down, the idle speed can be reduced to about 800r/min. The fault reader can detect the fault of the intake temperature sensor and the coolant temperature sensor line, and the trouble code can not be removed. After checking, throttle return position is normal, throttle sensor resistance, power supply voltage and line connection are all normal, all pipelines have no air leakage phenomenon. Replacement of air flow sensor failed to improve. Check ignition timing, normal. Take off the gear cover plate of the idling step motor, start the engine, observe the gear condition, rotate the gear automatically, the engine speed also rises, pull out the throttle sensor socket, the gear auto reposition, the engine restores the idle state. At this point, we could only suspect that there was something wrong with the ECU. Because the standard idle speed of the engine is set by the ECU, the ECU will be corrected to the rated standard idle speed in the air conditioning refrigeration and cold start, due to the increase of engine load, and now the engine idle speed automatically reaches 3000r/min, only ECU sends out the wrong signal to lift the idle speed, and the idle speed is stepping. The motor turns and opens the throttle, which indicates that ECU is defective. According to the above inspection situation, it is suggested that the owner change the ECU. At this time, the owner of the car said a new situation: the car was not long ago in the repair shop for full repair spray paint, the assembly of ECU, for some reason why ECU terminal broken two, had to find an old ECU installed. The maintenance technician asked the owner to bring the ECU of the original car to the ECU terminal socket, and the engine could start, but the working condition was not good. After checking, the two terminals are 12, that is, the temperature signal input terminal of the coolant, and the other 37, that is, the intake temperature signal input terminal. The function of the two terminals is to input the change signal of the sensor to ECU. ECU accordingly corrects the fuel supply at the start of the engine, and corrects the parameters of the fuel supply, the ignition time and the idle speed when the engine runs. If the change of electrical signals of the two sensors can not be restored, the engine will not achieve the best air-fuel ratio and the vehicle will not be able to operate normally. Since the two terminals have been broken and can not be recovered, the cost of replacing ECU is nearly 10000 yuan, which is difficult for owners to bear. Finally, the maintenance technician decided to repair the ECU. Carefully dismantle the plastic shell of ECU; find two insulated conductors of different colors, soldering solder in the connection between terminal 12 and 37, and the ECU circuit board; drill two small holes in the corresponding position of the ECU plastic shell, lead the two wires from the small holes, and seal the two small holes with the sealing glue to prevent the dust and the dust. Water gas intrusion; the input wire of the coolant temperature and the intake temperature signal in the wire harness of the ECU terminal socket is tied up with the corresponding wire drawn from the ECU. The engine was in good working condition after a trial run. After a week's call back, everything was normal, proving that the trouble was completely eliminated.