A Audi A8L car with a range of about 3200KM. User response: MMI of the vehicle is automatically closed during driving. Fault diagnosis: (1) check the vehicle found that the MMI can not be opened automatically when the vehicle is running or when the vehicle is re started. To do a guided fault query, the central module of the comfort system has 2 fault codes: the power supply voltage is too low. After checking the circuit diagram, check the relevant lines are normal. (2) continue the trial run and discover the new fault code: the media player in position 1 has no communication. After dismantling J794, the inspection line is normal, the fault diagnosis system is in normal, suspected that J794 may have intermittent failure, so try to replace a new J794 (the same configuration new car), after the replacement of the test fault did not appear, the decision is J794 fault, but second days when the customer receives the car, MMI system can not normal use, MMI system again can not normal use, the second days. It was at this time in confusion. (3) after rearranging the train of thought, the vehicle is found to have fault code again: optical fiber data bus break. So look at the optical fiber line, when looking at J794, when the disassembly and assembly suddenly MMI failure, move back and forth control unit will be restored, dismantled J794 found that the optical fiber plug has obvious bending marks, dismantling the accessory driver side sundries box observation, when loaded into the J794, the fiber is almost 90 degrees, leading to signal distortion. (4) reattach the plug optical fiber with adhesive tape, so that there is no bending condition in the optical fiber plug, forming a straight line. After the repair, there is no trouble in the trial run. The customer will give feedback after a week, and the fault will never happen again. Troubleshooting: refix the optical fiber line. Failure summary: a large number of lines in small space are crowded, resulting in deformation of optical fiber lines, resulting in failure.