Possible cause of fault

An optical (optical waveguide) or electrical (PCM2) open circuit in the ring
MOST switch in production mode
MOST switch incorrectly coded
An optically/electrically faulty MOST control unit is connected to the MOST
NOTE:
- The MOST optical waveguide is connected to the following control units:
- PCM2 as of model year 2003
- CD changer
- Amplifier
- Telephone/telematics
Fig 1: Identifying 8009 PCM2 - MMI Component Functions
Courtesy of PORSCHE OF NORTH AMERICA, INC.