Patient is a friends '00 Audi S6. ECU code 4D0907558. There is a persistent $9C9C unknown protocol error when trying to access ecu for fault code reading. Sometimes VCDS can connect as far as reading the soft coding and part numbers but quickly drops the connection with the $9C9C error before I can read fault codes, usually the connection doesn't even happen at all. I can access other controllers (ABS, Trans, Cluster, etc...) normally.

VCDS is a legitimate Ross-tech cable and software. Car and software is stock S6. So far I've tried it with a different ecu (100% sure working from my own car) with same results. The car starts and runs but there are some issues, likely with fuel trims and/or maf. I tried it with disconnecting the ABS controller also with no difference. Force K on engine from VCDS settings didn't make any difference. No aftermarket stereo, oem naviplus. The VCDS lead works with my own '00 S6 without any problems.

Any ideas or experiences? Maybe a wiring problem? Can wonky cluster cause this? Haven't yet checked if this car had a Jaeger cluster. ECU harness connectors look clean with no apparent water damage.

Yes, Win7Pro OEM install. Same laptop/vcds works fine with other cars, this problem is only appearing with this specific car.