Audi A3 2009 BCM Onboard Supply Unit Malfunction


Asked by Sep 06, 2017 at 03:24 AM about the 2009 Audi A3

Question type: Maintenance & Repair

Hi Everyone,

Am new at this, apologies if I break any protocols!

I need your help;

I have a mechanical issue with my Audi A3 2009 hatchback. Issue
manifested itself by having light failure warning on the car and hazard lights
turning themselves on when the engine is switched off.

I took it to the garage who 'cold rebooted' the vehicle by removing one of the
battery terminal. This caused BCM Onboard supply unit, Part No:
8P0907063D to blow on one of the input sockets.

I am currently seeking a replacement but my question is; what would have
caused the motherboard to blow? I am afraid of replacing it and the same
scenario happening.

Any advice will be highly appreciated.



2 Answers


Main reasons for failure in Engine Control Modules Corrosion and moisture: Corrosion or damage due to moisture is one of the main reasons for ECM failure. Corrosion can enter the ECM through the wiring harness and moisture can enter by a failure in the seals in the ECM itself. This happens over a period of time (5 to 10 years) due to the ECMs exposure to the elements. Fuel solenoid: The electronic fuel solenoid is also a main reason for failure in the ECM. The solenoid can cause a short in the ECM due to corrosion in the solenoid or the wire running from the solenoid to the ECM harness. The electronic fuel solenoid is located at the top of the fuel pump. The solenoid seems to fail due to corrosion as fast or faster then the ECM. If your truck is running fine then you shut your engine off and it won't restart then this is a good indication that the starter has shorted out the ECM. Injector wiring harness: The third thing that can cause failure in the ECM is the Injector wiring harness or the sensor wiring harness. Once again corrosion or breaks in the internal wiring in the harness can cause a short in the ECM or corrosion can enter the ECM through the wiring harness. Again this is caused by exposure to the elements. Grounding issues: Another issue that can cause failure in the ECM is poor grounding. This can be the result of loose or corroded ground wires to the battery or the frame. Starters: Replacing the starter with the wrong model starter is becoming a big problem with the Celect Plus ECMs. Many starter rebuilders will bypass the override sensor in the starter. The override sensor regulates the voltage going to the ECM so when the override sensor is bypassed you will get voltage problems in the ECM and it can generate fault codes or other problems. If you only started having problems with your ECM after installing a starter then the starter is most likely the root of your problem. Dead battery cells: Dead cells in batteries can cause failure in the ECM. Many times a battery is left in the rig long after a cell has died. This affects the grounding in the battery. Jump start: If the vehicle has been jump started recently and the cables were connected improperly this can cause a spike in your ECM and cause it to short out. A bad jump can also blow out 2 amps which are located between the ECM and the firewall.

2 of 2 people found this helpful.

Thank you COMETGUY61 for the pointers. Much appreciated. Would you mind pointing out the location of the ECM unit on my A3 8P 2009 model? I suspect the issue could have been caused by moisture/corrision as you have advised and I would like to confirm. Thank you once more.

1 of 1 people found this helpful.

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