Since 1998 I have been a very satisfied owner of an Audi A6 2.4 (1999 model).
Of recent, the car has been very embarrassing to me in that often when I drive it out and switched off the engine, it would refuse to start again as it goes into engine immobilization. The car is not fitted with any security device. It has done this three times within the last three weeks. And each time it had to be towed home.
Late last year I had to replace the transmission multi-function switch when the Tiptronic selector was not engaging. Subsequently, when the ignition key problem started, two (one wireless and one workshop) of the four original keys were condemned as bad; the remaining wireless and emergency plastic keys were re-coded before the car would become operational again. Now these two are given me the same problems as the condemned ones even though prior to their re-coding, they were never used and were with the original factory coding.