Two control modules communicate through a "two-wire Bus system"

• Switch off ignition.
• Disconnect both control modules.
• Check for open circuit in one of the Bus wires.
Fig 1: Schematic - Checking For Open Circuit In One Of The Bus Wires
Courtesy of AUDI OF AMERICA, INC.

• Check for short circuit in Bus wires one against the other.

Fig 2: Schematic - Checking For Short Circuit In Bus Wires One Against Other

Courtesy of AUDI OF AMERICA, INC.

• Check for open or short circuit to plus (B+) or to Ground (GND).

  If Bus-wiring is OK:

• Replace on a trail basis the least expensive control module and the one easiest to get to.
• If the communication is still not OK, replace the second control module.

Fig 3: Schematic - Checking For Open Or Short Circuit To Plus (B+) Or To Ground (GND)

Courtesy of AUDI OF AMERICA, INC.