I bought my 08 s6 needing work. I got the running and driving portion sorted and now am working on sorting the smaller issues out. Biggest one I am currently fighting is the mmi not working properly. Sometimes it will work for an entire drive, others it will either not turn on at all, or will cut on for a few seconds and shut off and not turn back on. I have tried an optical look on all of the accessories in the back, cd player and Ami under the glove box, Bluetooth module under the drivers seat and non of that helped. I’ve checked all of the fuses, and replaced the mmi circuit board. I only have vcds lite which only works on 06 and older so I haven’t been able to scan the optical system (trying to set something up). Anything else I can check before I have a chance to scan the car? Car does have an aftermarket stereo, so I am thinking about trying to put the oem back in, but I am assuming it was an issue prior to the swap and the reason the new radio was installed.
Hi there
I have similar experience with my MMI head unit.
I plan on replacing with an updated aftermarket unit installled.
Blue tooth Apple Car play touch screen etc.
New Head Unit professionally installed will be a great update warrantied etc
Hope this helps you!
Cheers
That is what I have, but without functioning mmi it still doesnt work. Screen works, Apple car play works. But no sound or car information. I could always completely bypass the mmi and run new speaker wires and amps. But that is a huge undertaking when the original system works great with the upgraded android screen (if the mmi worked).
You can maybe get the instructions and parts list from a new copy matching the aftermarket unit you already have. Then you can have yours uninstalled and reinstalled to attempt to find what was done incorrectly. Or you can just go back to stock and save all of the trouble.
Mine failed due to bad Bluetooth phone module. 10 dollar most ring fixed mine. With vcds you can run a most break test
I noticed on my, uts happening like last week alread, mmi making strange noise when playing music, but not all time, its like when FM frequency its not correct, but its happening also on usb music, probably something close to be “good by”
Anyone noticed same issue?
Only version of vcds I have currently is lite. My previous car was an 03 allroad so lite was all I needed. So I tried using an optical loop to test every module I could find that was part of the most system. No luck, so I am ordering full vcds so I can scan the car.
My issue was e1 media player 2 (aka cd deck). Everything seems to be up and working with that bypassed with a loop. Next project Is going to be getting the car on air ride.
Uuuuu… looking forward to be folowing progress on that.
Would you do a complete breakdown/documentation of the process? 
I may, I’m usually not one to document my stuff beyond basic pictures along the way. I have all of the stuff now. It will be a semi budget setup, and require some modifications. So it my not be aplicable to most.