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A 2008 Shanghai general Buick triumphant car with a mileage of about 90 thousand km. User response: the vehicle is sometimes unable to start, the starter operates, the engine has no signs of vehicle, the ignition switch is switched off, the engine is able to start. After receiving the vehicle: connect the fuel pressure gauge, turn on the ignition switch, and the fuel pressure is 400kPa, normal. At this point, the engine starts running smoothly. Using diagnostic instrument for self diagnosis, there is a historical fault code P1626, meaning the fuel anti-theft signal is missing. Looking up the maintenance manual, it is found that the fault code is set as follows: the engine control module does not receive the effective ignition key information in 1.5s or 2. 0s, or the engine control module is not activated after the ignition switch is opened. Clear the fault code and repeat the trial until the fault occurs. At this point, the fuel pressure is still 400kPa, which eliminates the trouble of the fuel pump and its circuit. Check the ignition system, no high pressure fire. The anti theft warning light of the driver's side door flashes, indicating that the fault is related to the engine anti-theft blocking system. The data flow is viewed according to the function path of the body, the interrupter, and the safety data. The ignition key signal is "no result", but normally it should be read out. It is inferred that the fault is on the chip of the ignition key. Checking the appearance of the ignition key is different from the ignition key of the original vehicle. The original ignition key is matched afterwards. Replace the ignition key and carry out anti-theft matching, trial run, trouble shooting. The engine anti-theft and blocking system consists of the following components: the ignition key, the anti-theft control unit (inside the antenna) installed on the steering column, the transceiver, the engine control module and the anti-theft warning lamp. When the ignition switch is turned on, the anti-theft control unit reads the ignition key password through the antenna and compares it with the stored password. If the ignition key is effective, the anti-theft control unit sends the permitted starting information to the engine control module, and the engine can start. If the ignition key is invalid, the fuel injection circuit and ignition circuit will be turned off and the engine will not start. There are two kinds of fault diagnosis pointcuts: one is troubleshooting, the other two is finding the cause based on the fault code. The current fault code can indicate the direction of the fault, and the historical fault can explain the failure. In short, combining fault symptom and fault code can shorten the time of maintenance and improve accuracy.