As a theft deterrent, Audi radios are programmed to lock up when power the deck has been disconnected. Disconnecting the car battery, therefore, or upgrading your system, can result in a stereo system rendered silent. The only thing your Audi's radio will show when you try to turn it on is "SAFE."
No sweat. Audi provides 4-digit codes, unique to each car and stereo, on a card in the jacket of each Audi's owner's manual. If you don't have your original manual, or if the card is missing from it, an Audi dealership should be able to look up the code for you, provided you have the radio's model and serial numbers, your photo ID and your Audi's vehicle identification number.
Locate your TT's owner's manual and look for the radio, audio or stereo subsection. On the inside flap, where your radio's model and serial number are printed, locate the four-digit code specific to your radio.
Insert the key into the ignition and turn it to the "on" position. There is no need to start the car. Turn on the radio and wait for the "SAFE" warning to appear on the deck.
Depress the "P.SCAN" and "RBDS" buttons simultaneously and continue to hold them down until "1000" appears on the screen. Immediately release both buttons.
Depress the radio's buttons 1 through 4 repeatedly until the code on the radio deck matches your car's specific code. The buttons correspond to a singe digit in the code. For example: The code 3-2-8-5 will require you to press the 1 button three times, the 2 button two times, the 3 button eight times and the 4 button five times.
Depress the "P.SCAN" and "RBDS" buttons simultaneously again, and hold them down until the deck re-displays "SAFE." Release both buttons. The radio should operate normally.
Only the original owner's manual will have the code specific to your vehicle -- replacement manuals will not work for this application. In such a case, you will need to call your Audi dealership to get the unlock code. Have the radio's model and serial numbers available, along with the TT's VIN. Accessing the radio model and serial numbers will require you to remove the decorative cowling around the radio and pull the deck partially out. If you are uncomfortable with this task, your dealer or mechanic should be able to check it for you for a nominal fee.
Take care to enter the code properly on the first try. Three failed attempts will result in the system locking up for an hour of key-on time: You'll have to leave the car switched on with the radio engaged in "SAFE" mode for 60 minutes before you get another chance. Depending on the system installed in your TT, three more failed attempts could result in the system locking up permanently. It will then need to be replaced.