ignition barrel

[MikkiJayne]

The immobiliser is coded to a transponder in the key. In the remote keys it's located in the front half next to the key blade itself and is either a green glass cylinder a bit larger than a grain of rice, or a small black rectangle about 12x3x5mm. If you swap this to the new key then it'll still be happy. The alarm and remote is coded to the board inside the key. Either you can swap this over, or there is a procedure to teach it new keys.

Have you considered a new barrel? Audi can supply a new barrel matched to the existing locks so you don't have to mess about with new keys and can retain a single key for all the locks. My fairly old copy of Etka shows it at £90, although that may have gone up considerably as a lot of D2 stuff has done. There are two variants - 107905855C and 107905855G. Audi should be able to tell you which you need based on reg number / vin and should also be able to get your key code from the vin too.