A Audi A6L car with a range of about 7500KM. Users reflect: the car's left front door cold light bar is not bright. Fault diagnosis: (1) vehicle cold light is not bright, direct replacement of the rear light or not bright, with diagnostic instrument detection also failed to find fault code records. (2) check left front door control unit coding is normal, measure cold light lamp plug to control unit wiring harness is normal. (3) check the model of the control unit and find that the left front door control unit is not the same as the right front door control unit model. When the chassis number is checked, the last letter of the left front is found not the same as the right front, and the same type of control unit is replaced. (4) after several tests, all the normal, check the maintenance record of the car has not been an accident, that is, in the wrong control unit in the factory fashion, replacement control unit after the failure. Troubleshooting: replace the same type of control unit. Fault summary: the wrong control unit type is 8 KO959793 M, and the correct control unit type is 8 KO959793 E.