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A red flag 7220E car with a mileage of about 120 thousand km. The car failed to start the car after a car wash. After receiving the car: try to start the vehicle, the engine is running, there are signs of car, but can not keep. Use diagnostic device for self diagnosis without fault code. Check the high pressure fire, the spark is strong, no problem. Measure the fuel pressure, within the range of standard values. Pull off the plug of the injector and check the working condition with the test lamp. When the starting light is on, the test light flickers, indicating that the fuel injection instruction is normal. Check engine timing mechanism, normal. It is reasonable to say that the engine should be able to start with the correct conditions of oil, electricity and timing. What is the reason? Check the air flow sensor and throttle position sensor. No fault was found. Pull off the plug of the air flow sensor and run the engine. Open the air cleaner housing and find that there are many drops in it, which seems to have been injected when washing the car. Clean up, plug in the plug of the air flow sensor, test the car, the trouble symptoms disappear completely, the maintenance work is over. The control unit of the electronic control system has the function of self diagnosis, but it can only identify the faults that exceed the set range, such as the sensor signal is beyond the standard value, circuit breakage, short circuit and so on. The water immersion air flow sensor has deviations in its signal, resulting in abnormal fuel injection, and the engine is difficult to start. After removing the plug of the air flow sensor, the engine control unit opens the fault operation mode and uses the throttle position signal to replace the air flow signal. The engine can start instead, but the power is restricted and the engine fault lights are lit.