WARNING: This page is about a different car, the 2001 Audi A6, 2000 Audi A6, 1999 Audi A6, and 1998 Audi A6. However, it is still accessible from the selected car via links, so may be relevant.
Check wire between test box socket 115 + 2 -pin connector terminal 2 for open circuit according to wiring diagram.