Go to:
http://ross-tech.com/
If you have a problem with one of the sensors or one of the receivers (there are 5 of each) you will have the problem you are describing (I did)
If you buy the cable, and download their program, you will be able to pinpoint the problem a lot cheaper than Audi will ever do. Might cost you $400 to $500 but this cable and program are invaluable.
Bill