Fault code 0901: Diagnosis/troubleshooting

NOTE: Visual inspection: In the next troubleshooting section, please check first that the affected pins on the rear convenience system control unit plug connection are neither damaged nor corroded. The wiring and plugs of the affected components must also be checked for external damage and correct contacts. Repair damaged or corroded pins if possible, otherwise replace affected pins. Repair or replace damaged wires.

NOTE: Sporadic faults: In the event of faults that occur sporadically, the wiring positions that are linked to moving parts on and in the vehicle must be checked for each system. Various statuses that can cause faults must be simulated with the help of the wiring diagram.

NOTE: Battery voltage: A battery voltage of between 11.4 and 14.5 V is required for the following troubleshooting procedure. This must be checked and provided, if necessary. On the vehicle electrical system, the voltage to the fuse (input side) must be guaranteed.

NOTE: In the left tail light unit area of the wiring harness there is a plug connector (rear cross-over) that must be included in the checks.

WORK INSTRUCTION CHART

Work instruction			Display OK	If not OK
1	Visual inspection	Switch on ignition. Engage reverse gear.	Left reversing light and right reversing light come on. =>Check bulbs for correct wattage and operating voltage (12 V/21 W). Go to step 2
2	Check wiring for short circuit to B+	Switch off ignition. Pull plug A off rear convenience system control module. Pull plug off left tail light unit. Pull plug off right tail light unit. Switch on ignition. Measure voltage between control module plug A, pin 22 and ground.	< 0.3 V Go to step 3	Check and if necessary repair pin 2 of the rear crossover plug connection in the left tail light unit area. Repair wiring. → End
3	Check wiring for short circuit to ground	Disconnect both plugs on the tail lights. Measure resistance between control module plug A, pin 22 and ground.	∞ Ω Go to step 4	Check and if necessary repair pin 2 of the rear crossover plug connection in the left tail light unit area. Repair wiring. → End
4	Check wire to left reversing light for open circuit	Measure resistance between pin 22 of control module plug A and pin 5 of left tail light plug.	< 5 Ω Go to step 5	Check and if necessary repair pin 2 of the rear crossover plug connection in the left tail light unit area. Repair wiring. → End
5	Check wire to right reversing light for open circuit	Measure resistance between pin 22 of control module plug A and pin 5 of right tail light plug.	< 5 Ω Go to step 6	Repair wiring. → End
6	Check wire to left reversing light and right reversing light for short circuit to ground	Measure resistance between control module plug A, pin 22 and ground.	∞ Ω Go to step 7	Check and if necessary repair pin 2 of the rear crossover plug connection in the left tail light unit area. Repair wiring. → End
7	Check ground supply of left tail light unit	Measure resistance between left tail light unit plug, pin 2 and ground.	< 5 Ω Go to step 8	Repair wiring. Check ground pin 41 and repair if necessary. → End
8	Check ground supply of right tail light unit	Measure resistance between right tail light unit plug, pin 2 and ground.	< 5 Ω Go to step 9	Repair wiring. Check ground pin 40 and repair if necessary. → End
9	Check left tail light unit	Measure resistance between pin 5 and associated contact springs in bulb holder. Measure resistance between pin 2 and associated contact springs in bulb holder.	< 5 Ω Go to step 10	Repair or replace left tail light unit if necessary. → End
10	Check right tail light unit	Measure resistance between pin 5 and associated contact springs in bulb holder. Measure resistance between pin 2 and associated contact springs in bulb holder.	< 5 Ω Go to step 11	Repair or replace right tail light unit if necessary. → End
11	Replace rear convenience system control module		→ End