I have had my own experience with my local dealer and the "diagnostic check" when they were unable to diagnose a faulty air con compressor and this issue went on for an age. I ended up winning my battle with them, however, I did learn that the dealers hands are tied in some respects when it comes to warranty claims. There is an extensive list of items that an extended warranty does not cover and before I bought mine, I made the effort to read the small print, that way I wont be surprised when they say no. However, having used the same main dealer for the past 15 years and the numerous MB's I have owned in that time, I would say that building a relationship with the service manager and getting them on side counts for something. When it comes the Mercedes rejecting warranty claims made by the dealer, it certainly helps to have them on your side.