MMI Dead, Screen Black, A8L D3

My Audi A8L MMI was dead, stuck at open, black screen, doesn’t close, every time I push a device button or reset the MMI, MMI initialize screen would come  on, and went black, while it was on, I could hear the relay clicking in the trunk under the back seat. 

VCDS scan would come up with: No communication with Head Unit, Optical Bus: open.

The culprit was the 5A yellow fuse was blown, I might have shorted the line somehow while working in the trunk. It did give me a great scare.

5A Fuse

fuse blown


  • I am so appreciative of your channel. Thank you for all that you share. I have an 06 A8L 4.2. I’m having the symptoms of Nav screen stuck open, sometimes shows the Audi MMI logo on screen for 5-10 seconds then goes dark. Frequently hear the rear dash clicking, etc. I don’t have vcds. I’m trying to determine where each fiber optic component for mmi is located, and what “order” the signal is sent. But mainly where each component is physically located. I’m aware of the control unit (or whatever it’s called) behind glove box, the cd changer, the navigation disc reader in left side of trunk, the Bose amp, and I guess a satellite radio module ? & Tuner? near amp also.. And I believe I read about a module under drivers floorboard carpet? I’m trying to use a fiber optic loop to isolate the issue. I’ve checked fuses in each side of trunk compartments as well as the 3 big flat Fuses above battery. I know just enough about electronics to know I don’t know very much !! The car was parked for several years and got it up and running several months ago. I did change battery and did not register it or code it in. But the radio was working for months. Then became sporadic. I did buy new mmi circuit board and change It because there’s no lights on control panel or working buttons other than the Start/Stop buttons. But still no lights work with new circuit board. I did unplug the fiber optic from amp and do see the blinking light but bypassing it with loop didn’t allow mmi interface to show up on screen. But since I saw the light on optic line blinking from amp area, does that, for example, mean the control unit or whatever it’s called behind the glove box is good ? Because the signal did manage to make it to the amp in the trunk? That’s why Ive been trying to find the location of components and sequence of Travel of the fiber optic path. For example: Nav unit >to> unit behind glove box >to> unit under drivers floorboard >to> amp >to> etc…etc Thank you for any advice you can share.
    Andy B from Louisiana

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