Sab did you ever get this sorted out? I just drained my coolant and remembered my 93 400SEL still has a heating problem, I am only getting heat from passenger side vent. Im tempted to try a new duo-valve. Do you guys think this is the problem? I just lost my job (laid off, probably permanently), and at $439, I cant afford to make a mistake. I wonder if it wasnt the problem if I could wash it off and return it to the dealer. I may try to run those diagnostic tests but my gosh, they seem very complicated. Any help is much appreciated!