I'm relatively new to Audis, but I've had lots of experience with cars that wouldn't start.

It is possible to have corroded connections on your battery that would allow enough voltage to run minor accessories, but not turn the engine over. Check that first, it's easy.

Next, look at the connections to the starter itself and any connections in-between. Any of them could corrode to the point of failure. Especially if you live in a salty area like me.

Also check this: Another battery question

You could also try to jump start the car, just to rule out any issues with the battery itself, although you said it is full.

85% of the non-starting problems I have experienced were from one of the issues above.

Good luck!

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2001 S8 - Racing Green Pearl Effect