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Dongfeng Citroen Seine 2. 0L car with a mileage of about 120 thousand km. The owner reflected that the vehicle could not start normally for several days in the morning. Fault diagnosis: first, the fault reading operation of the engine control unit assembly and the intelligent control box assembly is first performed with the diagnostic instrument PROXIA3, and no fault information that may cause the engine to be started is not found inside the two control units and continues to use the diagnostic PROXIA3 for the prevention of the intelligent control box assembly. It is found that the recognition and validation of the transponder is normal, and the matching between the intelligent control box and the transponder is normal. The engine control unit is in the unlocked state, while the engine unlocking and locking indicator on the vehicle's combination instrument is in the normal state, and the possibility of the engine can not be started normally because of the problem of the working state of the anti start system of the vehicle. A special battery state detector is used to check the voltage and working state of the battery on the vehicle. The result shows that the voltage value of the battery is completely in line with the engine starting requirement, and its working state is no problem (no need to be replaced or charged). (3) check the status of the engine speed position sensor and its working line (if the sensor and its line have problems, the engine control unit can't get the speed information, then the ignition time and the injection time can not be calculated, and the vehicle can not start normally). First, use multimeter to check the resistance value of the line, no problem. Continue to test the most open line of ignition switch voltage value, also meet the requirements. A new engine speed and position sensor assembly is replaced for engine startup test, and the fault phenomenon still exists. At this time, the maintenance technician considered that the failure was independent of the position of the engine speed sensor and its working line. (4) using the physical box and the MM6 LP electric spray system diagnostic line and the diagnostic instrument PROXIA3 to lead the working foot of the ignition coil in parallel, to detect the working waveform of the ignition coil under the ignition switch, and to compare the measurement results with the standard values under normal conditions, to remove all the spark plugs of the 4 cylinders and to carry out the visual inspection. All of them are grey and white. There is no carbon and burning loss in appearance. This can completely eliminate the possibility that the engine can not start the engine because of the engine ignition system. Check the state of the oil supply system of the vehicle, and use the special injector cleaning machine and the detector to check the working state of the 4 injectors, which fully meet the requirements (it can be opened normally and the amount of fuel injection is normal in unit time). Then check the state of the pump and the pressure of the oil supply line. The possibility of failure due to the failure of the oil supply system is eliminated. (6) check the working state of the intake system and use the PROXIA3 diagnostic instrument to measure the parameters of the intake system. It is found that there is a great difference between the intake pressure value and the normal value. Then the relevant working parts of the intake system are checked, and the intake pressure sensor and its working line are checked first, and a new sensor is replaced. As a result, the failure phenomenon still exists. Therefore, the resistance of the power supply and the wiring is checked, and the result is normal. Cleaning and checking vacuum tubes is also in line with requirements. Then the throttle body assembly was checked. When the throttle body assembly was dismantled to use a special cleaning agent to clean the mechanical part, it was found that there was an obvious leakage in the connection between the throttle body and the intake manifold, so the relevant parts were dismantled and the visual inspection was carried out to find the O shape of the throttle body and the intake manifold junction. The sealing ring is deformed, a new O - shaped seal is replaced, the throttle body assembly is rearranged in place, then the test operation is carried out, and the vehicle can be started normally. Maintenance summary: this fault is caused by the deformation of the O - shaped seal at the junction of the throttle body assembly and the intake manifold, which causes air leakage, resulting in a portion of the air entering the 4 cylinders of the engine not detected by the inlet pressure sensor, which will result in a clear mixture of gas in the cylinder of the engine. Engine ignition failed and failed to start normally.