If the motor stops suddenly: 1. First check whether the cam of the lens has turned to the limit position. Because the limit switch can disconnect the circuit when the cam turn to the limit position, making the motor unable to run in one direction. 2. If the cam of the lens is not in the limit position, check the wiring and power supply. Firstly, check whether the power supply is energized or whether the voltage meets the requirements. If there is the power and the voltage is within the required range, then check whether the solder joints are loose or fall off; If not, Find a well-confirmed wire to short-circuit each wire on the lens, and rule out whether the wires are disconnected one by one. 3. If none of the above situations occurs, short-circuit the limit switch of the lens with a cable to observe whether the motor can return to normal and eliminate the cause of damage to the limit switch. 4. Use a multimeter to measure the positive and negative output terminals of the motor on the auto-focus control board to check whether the voltage is normal under the condition of input and output. 5. Finally, when the motor is running, lightly touch the motor with your hand. If the motor is severely hot, it is likely to be blocked or damaged. Please cut off the power immediately, remove the motor and let it run idling. If it can rotate normally, please check whether there is any foreign matter stuck on the lens structure. If it cannot rotate normally, please replace with a new motor and test again.