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Dongfeng Citroen Picasso with a mileage of about 70 thousand km has a self integrated transmission car. Users reflect: the car's headlights automatically light, the front rain wiper motor has no regular automatic scraping, the air bag fault indicator light is often bright, but the car can start normally, other system fault indicator light is not abnormal. Fault diagnosis: (1) using the PROXIA diagnostic instrument to read the fault in the intelligent control box, and get the following fault information: the communication fault with the switching module of the steering wheel (local fault) (P, permanent fault). The PROXIA diagnostic instrument is unable to enter the diagnostic menu of the steering wheel lower conversion module during fault diagnosis. This indicates that the diagnostic instrument can not diagnose communication with the steering wheel lower conversion module of the vehicle through the diagnostic connector. Because the headlights are lit automatically and the wiper motor is automatically moving, these actions are the performance of the downgrade model after the failure of the typical VAN body network, and read the fault of the module under the steering wheel from the diagnostic instrument, so it can be judged to be related to the VAN body network. 2. Replace a new steering wheel down conversion module assembly, failure phenomenon did not disappear, using the PROXIA diagnostic instrument to read the fault code, the fault still exists, and the fault code deletion also has no effect. This indicates that the steering wheel down conversion module itself has no fault. Continue to check the state of F4 insurance in the intelligent control box, which is related to the power supply of the steering wheel lower conversion module, and find that it has been blown. Change a new 20A insurance, failure to solve the problem, indicating that there are other problems. (4) connecting the diagnostic wiring harness of the intelligent control box with the 126 route diagnosis junction box to check the voltage values of the feet associated with the steering wheel down conversion module in the ignition switch, of which the voltage of the 3 foot of the 16 VNR connector is the 2 feet of the 12V 916VGR connector and the 28 and 30 feet of the 40VBA connector are 3., respectively. 7V and 1. 2V. This indicates that the state of the working end related to the pins of the steering module under the intelligent control box is normal. (5) measuring the resistance value between 28 and 30 feet of the intelligent control box 40VBA connector, which is 2. 07K Omega, in the normal range, this shows that there is no problem between the connection between the DADA and the DATA / network of each parallel control module of the VAN body network. The following information can be obtained from the above information: the fault is from the line between the intelligent control box and the steering wheel down conversion module. The 6 VGR connector of the steering wheel down conversion module is pulled out and the connection wire harness of each foot is measured. It is found that the resistance value between the 4 feet and the body lap is infinite, and the normal value should be about M. The resistance value measured by the foot related connection line is in line with the normal state. Continue to check the line related to this leg: from the state of the switch module to the MCV00 line of the MC3 tie point, it is found that there is a broken fault on the line of the iron lap. After welding it with the electric soldering iron and conducting the insulation treatment, the fault code reads and the fault code is deleted by the diagnostic PROXIA. The barrier code has been eliminated. After the ignition switch is opened, the car is normal and the trouble is completely eliminated. Fault summary: this fault is due to the breakup of the lift line of the steering wheel down conversion module. The steering wheel down conversion module fails because of no iron lap, and the vehicle enters the VAN body network downgrade mode. The car shows the following downgrade operation mode: the headlights automatically light, the front rain scraper motor has no regular self. Wiper, air bag fault indicator light. Troubleshooting the failure of the grounding points is solved.