The mileage is about 40 thousand km, the engine type is BYD473QB, the BYD L3 manual comfort car matching the 5 gear manual transmission is in 2011. User response: the car sometimes starts to gear up when it starts, and is unable to start at this point. Inspection analysis: according to the analysis of the failure phenomenon, possible reasons include: ignition switch fault; starter fault; starter gear or flywheel gear ring wear; line fault. In line with the idea of simplicity and complexity, the maintenance personnel first check the starting line, without exception. Disconnect starter's terminal 50 (starting relay terminal), connect a 12V25W pilot lamp, and start the test lamp to normal lighting. Remove starter, check gear and flywheel gear ring, and find no wear and loosening. In desperation, the replacement of the new starter test car, after repeated failures will still appear. In summary, it seems that all possible reasons have been checked, but the trouble has not been solved. At this time, the maintenance personnel boldly envisaged whether the current was not large enough to cause the starter to be in the wrong place. So the 50 terminal of the starter is disconnected, and a large load is selected in the starting circuit, and the 12V255W test lamp is selected. After multiple starting tests, it is found that the engine can start normally when the luminance is normal (at this time the current is larger), and when the light of the test lamp is dark (the current is small), the phenomenon of tooth beating will appear. This confirms that the fault is due to the large and small time when the starting current is small. Further check the cause of the abnormal starting current. Disconnect the starting relay negative electrode to the wire of the anti-theft control unit, directly connect the tie - in point, many tests can start normally, it seems that the problem is inside the burglar control unit. Apart from the anti-theft control unit, it is found that the inner circuit board is corroded. The cause of the failure is found to be due to the internal corrosion of the anti-theft control unit, which causes the incipient circuit to be undesirable. The current of the starting relay arrives at the electromagnetic coil is too small, resulting in the inabsorption of the starter, and the gear can not be fully meshed with the gear ring of the flywheel, resulting in tooth beating and the inability to start. Troubleshooting: replace the new body anti-theft control unit and match it, troubleshooting.