If you cannot determine a cause for the malfunction, e.g. "power train data-BUS faulty", check whether one of the control modules may be responsible for the DTC

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Audi S8, 2003 Audi A8, 2002 Audi S8, 2002 Audi A8, and 2001 Audi S8. However, it is still accessible from the selected car via links, so may be relevant.

All control modules that use the CAN-bus are still disconnected. Ignition is switched off.

Connect one of the control modules.
Connect VAG 1551 scan tool. Switch ignition on, and erase the DTC memory of the control module you just connected. End scan tool output using the "End Output" function 06.
Switch ignition off and then on.
Leave ignition on for 10 seconds. Then, read the DTC memory of the control module you just connected.
If the DTC "power train data-BUS faulty" is displayed, replace the control module you just connected.
If the DTC "power train data-BUS faulty" is not displayed, connect the next control module and repeat the procedure.