Two control modules communicating via a CAN-Bus

Switch off ignition.

Fig 1: Disconnecting Both Control Modules
Courtesy of AUDI OF AMERICA, INC.
Disconnect both control modules.
Check for open circuit in one of the bus wires
Check for short circuit between the two bus wires

Fig 2: Checking Short Circuit Between Two Bus Wires
Courtesy of AUDI OF AMERICA, INC.
Check for short to positive or short to Ground (GND) in one of the bus wires.

Fig 3: Checking Short To Positive Or Short To Ground (GND) Circuit In One Of Bus Wires
Courtesy of AUDI OF AMERICA, INC.

If no malfunction can be found in the bus wires:
As a trial measure, replace whichever control module is easier (or less expensive) to change.
If communication between the control modules via the bus is still not working properly, replace the second control module.