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.
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I tried to turn on the hidden menu so I could activate the graphical display for the parking sensors. After I did this I reset the MMI, and now it won’t turn back on. The fuse you describe here is good and VCDS can’t communicate with the controller.
Not sure where to from here, I disconnected the battery to see if it just needed a reset.