A Audi A6L 2.5 car with a mileage of about 20 thousand km. The headlights of the car occasionally alarm. Fault diagnosis: headlights occasionally alarm, with AVS6150 detection, there is a headlight control unit can not reach the failure. Because of failure, check the control unit power supply, tie in and CAN line first. Because this fault is an accidental fault, the fault frequency is not very high, and the failure occurs occasionally during inspection, but the time is not long, so the fault is not found in the measurement line. The replacement of headlight control unit and gateway failure can not be excluded, so the fault is locked on the line. The CAN high and low line short circuit was measured at the instant of a fault reappearance. It was found that there were joints between the two CAN wires halfway, and the insulation skin was not covered after pressing. Troubleshooting: the CAN line pressure joint package insulation skin. Fault summary: this kind of contingency fault suggests that after changing the possible control unit, it will lock down the fault before you have confidence to check the line.