Audi ecu gets the key from answering 502 request, and adapts it to work.
Requests on tested audi are 04 04 FA 04 02 04 00 04 on 502h.

Original PSG changes key when sent a sequence of messages from 4A8h CAN ID.

Only segment of the request that immobilizes the engine start is 0xFA04 (on tested vehicle). known key

Solutions to immobilizer:
VCDS pump adaptation with PIN. Herlicson PSG sent with generic key.
Cloning original PSG key. For this solution, we need to repeat the 4A8h sequence as well as the 502h requests and clone the last sent 516 key value. 

For cloning the original PSG key, we should test in other audis with immobiliser if the 4A8h sent sequence is the same.
Other addresses on the 502h also return unknown values: FC04 F204 A004 A204 A404 etc.

Current known 4A8 sequence store at pcan-explorer file, Capture_audi_immobiliser_key.trace