A Audi A6L car with a distance of about 83 thousand km. When the vehicle starts, the reverse gear is normal, the forward wheel is locked about 2S, and the brake is continuously pressed. It feels that the brake pedal is hard and the hand brake is normal. Fault diagnosis: (1) detect the trouble free code display with VAS5052. At that time, it was considered whether the ABS hydraulic system was damaged, so the ABS pump was deflated, and the system was stored on the gas, but the failure phenomenon remained. Then we disconnect the ABS line, that is, the failure of the ABS system will disappear when the system does not work. (2) check the oil circuit of the electric plug and brake oil, all are normal, and then replace the ABS pump, the trouble is still. (3) it is normal to think that this car is pulling the hand brake. It is doubted whether the hand brake control unit is a problem. After reading the data stream of J540, it is found that the third data in the data stream 2 shows 36 degrees, which is about the horizontal angle. But the vehicle is in a horizontal position, indicating that the J540 control unit misreported the vehicle on the ramp, causing the above fault. (4) replacement of J540 and troubleshooting. Troubleshooting: replace J540. Fault summary: when you encounter problems, you should look at it in many ways, and read the data flow to find out a lot of reasons for the failure.