All VW dealerships should have a special tool called a VAS 5051. There is a smaller, mobile version of this tool that is called a VAS 5052. The 5052 is more easily portable and can be taken in the car for test drives, etc.
I mentioned use of this tool in a thread that CoolWhiteWolfsburg started, where he mentioned fuel gauge problems in a Phaeton. That thread is here: V8 Gas gauge problems.
I think it is useful for us, as Phaeton owners, to be aware of amazing capabilities of this diagnostic and programming tool. Below are a few photos that show the tool, the kind of information it can pull out of the car, and how it is used.
VAS 5051 plugged into my Phaeton
VAS 5051 booting up
Where it connects
It's a two second job to connect the 5051 to the vehicle. No dis-assembly is required.
Note the choices available to have the computer complete 'Vehicle Self-Diagnosis', and also to help the technician find the problem via "Guided Fault Finding".
Example of Complete Self-Diagnosis (in progress)
This is my car. The 11 faults in the air conditioning and heating electronics were all traced to a single bad ground connection.
The tool will identify the exact location of a fault with remarkable precision
This particular fault was causing my nav system to occasionally lose track of where it was when driving in underground garages, although I never noticed any problems with the speedometer.
Here is where programming changes are made
This screen shows the coding for my instrument cluster before changes were made to get rid of the seat belt warning chime and the 'driver door open with key in ignition' warning chime. There is more information about this programming change at these threads: How to get rid of the seat belt warning message and chime and How to get rid of the "Key in Ignition" warning chime (Changing Country Code)
I think it makes sense to take your Phaeton to a dealer who has the correct tools. A VAG-COM cannot substitute for a VAS 5051 - it lacks the guided fault finding and the self-diagnosis capability. I'm not putting down Ross-Tech's product - in fact, I've ordered a VAG-COM for my own home use - but realistically, there can be no comparison between a $400 product that runs on a laptop, and a $20,000+ product that is made by VW, strictly for VW's.