A motor vehicle with a mileage of about 50 thousand km, equipped with 4 automatic transmission in 2007, FAW Mazda 6 2. 0L sedan. Users reflect: the car will occasionally alarm after the car locks, must be used remote control or key locking / unlocking once, the vehicle can be restored to normal, and the fault has occurred two or three times. Fault diagnosis: when the vehicle enters the factory, it can lock the car normally, and it fails to report to the police repeatedly. In view of the fact that the car anti-theft system fault can not be diagnosed by the diagnostic instrument, it can only be analyzed according to the control principle of the anti-theft system. In the car's anti-theft system, when the door and the trunk cover are closed, the four door lock lock signal, the luggage lock lock signal and the engine room cover lock signal are given to the control unit. When the control unit receives the key or remote control locking signal, the system enters the state of burglary. If any vehicle door, trunk lid lock or engine room cover lock is unlawfully opened, the vehicle will report to the police. The vehicle alarm after lock up indicates that the lock opening signal is input to the control unit. Because it is an accidental fault, check the above parts one by one (gently shake the door and trunk cover), but the fault never appears. However, it was found that the trunk lid lock could not be controlled when the remote control was used to lock the vehicle. Normally, the trunk lid can not be opened after the vehicle is locked, and the trunk cover can always be opened. According to the control principle of the vehicle anti-theft system, this failure point is not the direct cause of vehicle alarm, but it needs to be eliminated. For this reason, it is decided to repair the trunk lid lock first. The unchecked luggage lock lock found that a linkage lever was less on the lock actuator so that the remote control could not control the cover lock of the trunk, and it had been dismantled. After further inspection, it is found that the lock lock switch (key signal of the alarm) is shorter than the metal spring of the normal switch, which causes the lock to be popped out sometimes, so the lock open signal will be input to the control unit, which causes the vehicle to alarm by mistake.