A 2008 Porsche Cayenne with a mileage of about 75 thousand km. Users reflect: the car is stopped for one night, the vehicle can not start second days, open the ignition switch to the start gear, the starting machine does not react, the instrument is displayed, the sound of the horn is obviously low, the obvious sign is that the battery power is low. Fault diagnosis: arrive at the rescue site, use other vehicles to connect, start the vehicle, find the fault is still, on the dashboard display "Warning Brake Proprotioning", "Press Brake Pedal" prompt information, while the instrument displays the ABS alarm light, the parking brake lights flicker constantly, and the shift lever switch also Under the condition of stepping on the brake pedal, it is not possible to move out of the P gear position. When the brake lights are working at the rear of the vehicle, it is found that the brake lights are not lighted. Use PIWIS Tester to enter the vehicle diagnosis interface, enter the DME control unit with fault memory P1606 communication, ASR control unit, P0503 speed sensor A discontinuous / irregular / high fault information, and return to the diagnostic interface to enter PSM control unit, display the unfound system; enter the instrument control unit: 01316, PSM control unit, No signal communication, fault type open circuit, fault status exists. 01299, the gateway control unit, no signal communication. Entering the Tiptronic control unit has the fault code 01316. Entering the Gateway control unit also has a fault memory 01316. The horizontal control system unit also stores 01316 fault memories. At this point, the failure phenomenon that the engine can not start can show a basic fault path, because the brake pedal switch signal is not sent by the PSM control unit through the data bus system through the gateway unit through the data bus system, and causes the cascading failure caused by the brake signal and the PSM unit fault. According to the same 01316 fault code phenomenon of multiple control units, and PSM control unit can not communicate with Tester, first check the PSM power line, and the fuse is normal. Check the A4 foot of the PSM without voltage, pull down the fuse to measure the resistance between the pin and A4, and result the F38 safety bit is opened to the PSM control unit A4 foot. The wire harness circuit breakpoint detection is carried out, and the phenomenon of virtual connection is not found. The wiring harness between the PSM control unit and the body wire harness installation hole is detected with a segmented detection with the diagnostic line. Switch on the ignition switch and shake the wire with your hands. The measuring unit A4 sometimes does not measure the voltage. It proves again that there is a fault in the wiring harness. In order to verify whether it is the phenomenon that the vehicle can not start the vehicle caused by the fault, it is decided to use the diagnostic line to cross the F38 safe to the PSM control unit A4 foot, close the ignition switch and connect the cross line, install the electrical connection of the control unit, install the DME control unit (in the upper part of the PSM control unit) and open the ignition switch With PIWI STester into the diagnosis interface, it can enter the PSM control unit, and can remove the fault memory, return to the vehicle diagnosis interface, start the engine, the engine is started smoothly, and run smoothly. At this point, the fault point is completely found. Troubleshooting: stripped the wiring harness insulation pipe and found that the line near the right drain pipe was soaked in water and severely oxidized. Repair lines to dredge drainage pipes, troubleshooting.