The mileage is about 147 thousand km, and 2007 Volkswagen sedan cars equipped with manual transmission. Users reflect: the left rear door glass of the car can not be lifted, whether it is the front door glass lifting main switch or the back door glass lifting switch can not control the glass lift. In addition, the left rear door lock is invalid, and the door lock of the left rear door can not be controlled through the remote control and the central locking switch of the left front door, and the door lock can not be locked normally and locked. Fault diagnosis: after the connection, the V. A. 55053 diagnostic instrument is connected to the vehicle to carry out a comprehensive test. When the central control system (address code 09) is detected, a fault code is found to be 01333, and the fault is J388 without signal. When detecting the CAN gateway (address code 19), there are 01333 fault codes stored. And when the left back door (address code 62) system is detected, the system displays a fault free code, but the component number is Noresponse, not the conventional I KO 959 703P, the component is the ERRORLIN SLAVE 0 character, not J388, the code part is 0000000, and the WSC code is also 00000. Through these fault information, it can be seen that both the central control unit and the gateway system control unit have not received communication signals from the left backdoor control unit, because the vehicle is a vehicle produced in December 2007, and the communication mode of the left back door control unit of the car is connected to the left front gate control unit by the LIN line. The left front door control unit, the central control unit and the gateway control unit are connected through the CAN-BUS bus. The glass control main switch and the middle lock switch of the left front door pass the left front door control unit to the left rear door control unit by using a LIN line transmission mode, and the left rear door control unit is controlled by the left front door control unit. The work of the left rear door glass lifting motor and the work of the door lock motor are made. It seems that this LIN line is very important. A line serves not only as a control signal but also as a communication unit between control units. The central gateway and the central electric control system and the left front door control unit are all normal through the diagnostic instrument, which shows that the CAN-BUS communication network is not a problem. The results show no fault code storage when the left back door control unit is detected. It does not mean that there is no fault in the left rear gate control unit system. From the detection phenomenon, there are three possibilities: one is the LIN network between the left front gate control unit and the left backdoor control unit, which is a circuit breaker or a short circuit to the ground. The other possibility is the problem of the left back door control unit itself; and another possibility is that the power supply part of the left rear door control unit has problems. Due to the lack of the circuit diagram and technical data of this vehicle, the connection plug between the left rear door and the B column is dismantled and measured. It is found that the power supply and the iron line are normal. The inner lining of the left door door is disassembled and the control unit part is measured, and the power supply is also normal. Because there is no oscilloscope and can not measure the signal in the UN line, it can only measure whether the LIN line between the left front door A column and the left rear door control unit is on and whether the ground is on the ground or not. When using the multimeter, it is found that the LIN line is connected, the front door lining is dismantled, the LIN line between the control unit plug part and the left rear door control unit is measured, and it is found that the LIN line between the left front door control unit and the A column plug is broken. It seems that the problem has been found. The circuit was cut open and a purple white line was broken. This line is the LIN line between the two control units, the LIN line is repaired, the communication is normal, the part number and other information of the control unit can be measured, but the left front door main control switch can not control the glass lifting of the left rear door, and the left rear door switch can not control the glass. This is strange. The power supply part is normal, the LIN line is normal, and the communication is normal. Is the left rear door control unit damaged? Only the left rear door control unit can be replaced. Replace and control the left rear door control unit, trial run, troubleshooting. The main switch of the left front door can normally control the lift of the left rear door glass. The left rear door switch can also control the lift of the left rear door glass. The left rear door lock can also work normally. The remote door lock can also lock the car completely. At this point, the fault is completely eliminated. Through such failures, network control is becoming more and more popular. Without data or equipment support, it is difficult to find the problem quickly.