As an antitheft measure, every factory-installed radio in an Audi has its own unique four-digit security code. If your radio ever goes into Safe Mode due to a car battery change or some other circumstance, you must enter the code to unlock it. You can't change the code, and if you don't have it, you need to contact your Audi dealership.
Call your Audi dealer and let them know your radio's model and serial number. Popular radios installed in Audi's include Rothenburg, Wiesbaden, Delta, Gamma, Concert, and Symphony. Your dealer will look up your 4-digit security code.
Press the 2-button combination on your radio that initiates the programming mode. This will vary depending on your radio, but on a Concert II, for instance, the combination is "SCAN" and "RDS." Hold the buttons down until you see "CODE," followed by "1000," on the radio's display, and immediately release.
Press the "1" preset button the number of times equal to the first digit in your radio code. For instance, if the code is 9876, you would press the "1" button nine times. You should see this number on your radio's display as you are entering it.
Press the "2" preset button the number of times equal to the second digit in your radio code. If the code is 9876, you would press the "2" button 8 times. Likewise, the third and fourth digits of your radio code correspond to the "3" and "4" preset buttons, and are entered in a similar manner.
Press the 2-button combination again, until you see "CODE" followed by "1000" on the radio's display. If your radio does not work at this point, and the radio's display reads "SAFE" when you press a button, leave the radio on with the key in the ignition. The radio will reset after about an hour, and you can try again.
Do not attempt to unlock your radio while driving.