A Mercedes S500 car with a 220 chassis. Car owners reflect: the car can start the normal driving, but when driving, steering lamp, steering column regulator, seat adjuster, wiper, smoke device, air conditioner, horn and so on can not be used. Troubleshooting and Troubleshooting: the test found that when the remote control is used, the door is responsive and can be locked and unlocked smoothly, and the steering lamp flickers normally. In addition to the car owners described the fault phenomenon, the car also has SRS alarm lights lit. After the ignition switch is switched on, the fault electric appliances described by the owner can be used normally, but after the vehicle starts, faults appear, and all the electrical appliances are out of order. Trial and error will occasionally occur when the vehicle starts. First, find the maintenance manual, find the fuse instructions, the fuse box under the rear seat of the car, the related fuse and relay and peripheral circuit in the two fuse boxes in the engine room. Then the X431 fault diagnosis instrument is used to detect the vehicle electronic control system by computer, and multiple fault codes are transferred. From the explanation of the fault code, most of the fault codes represent the failure of the CAN-Bus (controller LAN bus) communication failure and the control of the low power supply voltage of the component. After the fault code is printed and recorded, the fault code is deleted. After part of the invalid code is deleted, the vehicle is started and the fault remains unchanged. The fault codes are read again. The fault codes are: "5019 control unit and central gateway communication fault", "low voltage of B1001 control unit", "abnormal B1013 terminal 15", "B1342 input signal terminal 15 bad", "B1012 export KI 15 short circuit". From the analysis of the trouble code, it is possible that the question of doubt is on the comfort system control unit SAM (body computer). Based on the previous experience of maintenance of the vehicle network system, it is found that most of the failures like this "large area of mass death" have been caused by the CAN-Bus network. Also because the 220 chassis of Mercedes Benz is prone to the phenomenon of stagnant water behind the rear seat, it will cause the SAM computer to be soaked, so the key SAM is checked. After careful examination, the SAM has no water after determination, but it is found that both the former SAM and the post SAM power supply have different degrees of corrosion. After maintenance and treatment, the measurement voltage is more than 12V, and the fault is still re tested. The Mercedes Benz S500 models have "front left SAM", "front right SAM" and "post SAM", which are used to collect signals from each switch and sensor, and transmit the control instructions to the various executers. According to previous work experience analysis, the probability of computer damage is not large, several computers at the same time the probability of damage is smaller. After reading the circuit diagram, the horn and the right headlight light are controlled by the front right SAM; the wiper, seat adjuster, direction tube column adjuster and left head lamp light are controlled by the front left SAM; the taillight and related parts are controlled by the rear SAM, and the computer and the computer exchange data through the CAN-Bus. The actuator control of the wiper, steering, seat, steering wheel and so on is carried out through the control function of the X431's actuating element. The control instructions are sent through the control instructions of the X431 to control each actuator, and it is found that all the executive components are able to act according to the instruction. As a result, the power supply and control loop, SAM computer and related lines and actuators are all clear. In that case, why can't every system work? The author reorganizes the idea again, starts from the fault code, analyzes the fault phenomenon, finds several power failure codes, all points to the No. 15 power supply circuit, and the No. 15 power supply is the signal provided by the ignition switch. At this time, suddenly think, Mercedes Benz ignition switch EIS fault frequent situation, suddenly open, immediately to the EIS test, found the ignition key signal lost. After changing the ignition key and re matching, remove the trouble code, repeat the test and troubleshooting. Comment: the author of this article is basically correct in removing the fault of the car. The meaning of the fault code can easily be associated with the problem of CAN-Bus (controller LAN bus) communication problem or the comfort system control unit SAM (body computer). But if the author can test the car immediately after reading the fault code, it may be able to judge the comfort system control unit SAM, and the problem should appear on the control signal. As long as the meaning of the fault code is analyzed, all the fault codes point to the 15 power supply circuit. At this point, the EIS and circuit related to the No. 15 power supply should be checked.