A 2003 Guangzhou Honda accord car is equipped with a 2 4 Li V engine. According to the owner, the engine idling is unstable, and the engine fault lights occasionally light up. The maintenance personnel found that the engine idle speed changed from 700 to 1000r / min, and the engine fault light on the dashboard was lighted. When the engine speed is raised to 1800r / min, the speed of the engine can only be raised before the accelerator pedal is stepped up. When the engine speed reaches 1800r / min, the engine speed of the accelerator pedal can be improved smoothly. The engine fault code is used by the Honda HDS diagnostic instrument. There are 2 fault codes P0511 and P2195. The meaning is the idle air control (IAC) valve circuit fault and the post air fuel ratio (A / F) sensor signal. Clean up the fault code and carry out the idle learning, the road test, after a long time test, there is no idle idle and light light phenomenon, the maintenance personnel think that the previous fault code is only accidental fault code, the engine system is already normal, and then handed to the user. On the second day, the car went into the factory because of the same malfunction. The engine idling was more serious, and the engine fault light was lit again. The fault code P0511 and P2195 were redeployed with the HDS diagnostic instrument, and the data flow was examined. There were 4 data anomalies: (1) the angle of the throttle position sensor changed from 9% to 15%. 2. The value of idle speed control valve varies from 9% to 18%. 3. The injection time varies from 4.67 to 10.00ms, but 0ms will appear irregularly, that is, the fuel injector does not spray. (4) the ignition advance angle changes at 7~14 degrees, but it will appear irregularly two degrees and 10 degrees. From the data flow analysis, the abnormal data are all related to the fault code P0511. According to the troubleshooting process of P0511 in the maintenance manual, there is no problem on the line connection, the idle control valve is replaced, the fault code is cleared and the learning of the idle speed is carried out. Look at the data flow, the data is normal and the engine is running normally, but after a section of the test, the engine appears idle failure. Maintenance personnel tried to replace throttle body, engine control unit and gasoline pump assembly, but no trouble was removed. There is no way to proceed with the maintenance work. The author summed up the previous maintenance work, and combined with the failure phenomenon, the author thinks that the cause of the fault should be in the electric control part of the engine. It should be the signal deviation provided by a certain sensor, which causes the engine control unit to not send out the correct execution signal. From the fault code analysis, the idle speed control system is more likely to appear problems, but has checked the related lines, also changed the idle control valve, no problems have been found, why will a period of time on the road will be malfunction? In my opinion, it is possible for the vehicle to cause bad contact in the course of driving due to vibration. The circuit diagram of the idle control system is found. The black / red signal line in the 3 wire plug on the idling control valve is connected directly with the black / red signal line in the wire harness connector A12 on the engine control unit with the cross wiring, and then the road test is carried out, and the test car has no fault for a long time. Remove the jumper, and connect the black / red line in the connector A12 to the body to connect the multimeter to observe the voltage. The static voltage is 11.80V, which is equal to the voltage of the battery, but after a period of time on the road, the voltage suddenly turns into 0V, then the engine fault light is lit and the idle jitter appears. Through the above examination, it can be determined that there is a bad contact between the black / red signal line in the wire harness connector A12 on the engine control unit and the signal line of the idle control valve. Once again, the idling control valve line is thoroughly checked. When the 33P fast connector between the engine harness and the engine control unit wire harness is pulled down, the author finds that the 1 pins in the connector have been back to the plug, so that the pins can not be completely inserted into the plug hole, but only with the jacks. Surface contact causes poor contact between wiring harnesses. Refer to the maintenance manual, the pin connected to the top is the line of idle speed control valve. When the pin is re fixed, the connector is installed in place, and the commissioning trouble is completely eliminated. After knowing the cause of the failure, the user reflected that when the car had a mileage of 30 thousand km, a collision accident in the front of the body had been disassembled in the other repair plant, and it might be a hidden trouble left by the improper operation when the plug was inserted.