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

WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2005 Audi allroad, 2004 Audi allroad, 2004 Audi A6, 2003 Audi allroad, and 2003 Audi S6. 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 VAS 5051 tester. 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 "powertrain data-BUS malfunctioning" is displayed, replace the control module you just connected.
If the DTC "powertrain data-BUS malfunctioning" is not displayed, connect the next control module and repeat the procedure.