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The mileage is about 6000 km, the Audi A6L car with 01J automatic transmission. User response: sometimes there will be a "gearbox failure" warning message on the dashboard of the vehicle. Fault diagnosis: when the fault occurs, first connect VAS6150B to read the fault memory of the automatic transmission control unit. The fault code is: 01872 P0750 000---- switch valve 1 solenoid valve 1-N88 fault; 05975P1757 000---- voltage supply open circuit; 06168 P1818 006----- pressure control valve 2-N216 line electrical fault. Then the data flow of the automatic transmission system is read. The power supply voltage of the automatic transmission control unit is 14.2 V. According to the analysis of the above fault code, the automatic transmission control unit fault is determined. The automatic transmission control unit is replaced, and the vehicle is out of the factory. 1 months later, the car came into service again, reflecting the failure. Once again connected to VAS6150B for testing, the above 3 fault codes are tuned. Again, a serious analysis of the fault is that the solenoid valve N88 and the pressure control valve 2 (N216) are integrated inside the automatic transmission, but the fault code for the replacement of the automatic transmission control unit still exists, and the fault code 05975P1757 000 voltage supply opens not only the power supply of the automatic control unit itself, but also the power supply of the automatic speed regulator control unit. It has a great relationship with the external line. According to the circuit diagram analysis, the 30 line of the automatic transmission control unit is powered by the main relay (J271), and the left relay box of the dismantled drain tank is checked. It is found that the main power supply line is loose and the nut of the main power line is not screwed up, and the J271 is powered by the main power supply line. Troubleshooting: replace the left side relay box power supply bracket.