2003-2004 Audi A4 Cabriolet Radio Communication

Radio system General information CAUTION! Before beginning repairs on the electrical system: Obtain anti-theft radio security code if necessary. Switch the ignition off. Disconnect battery Ground (GND) strap. On vehicles equipped with Audi Telematics by OnStar , switch-off the emergency (back- up) battery for the Telematic/Telephone Control Module prior to disconnecting vehicle battery. Use VAS 5051 Vehicle Diagnostic, Testing and Information System in mode "Guided Fault Finding" After reconnecting vehicle battery, re-code anti-theft radio if necessary and check operation. Also check operation of clock and power windows according to Repair Manual and/or Owner's Manual. After reconnecting vehicle battery on vehicles equipped with Audi Telematics by OnStar , switch-on the emergency (back- up) battery for the Telematic/Telephone Control Module. Use VAS 5051 Vehicle Diagnostic, Testing and Information System in mode "Guided Fault Finding"
Note: Further measures after reconnecting the voltage supply: Repair Manual, Fuel Injection & Ignition, Repair Group 24 Before troubleshooting or servicing, technicians must be familiar with the functions and operation specifics of the applicable standard or optional radio system. Always read the radio owner's manual and review all radio, tape player and CD player/changer functions.

Source