It seems that there is a problem with the "Smart Key" on the new C, and it is being replaced with an upgraded key. Actually, it has already been replaced with one upgrade, and another is being released in a week or two.

My key functioned fine at first. Then it started to do the "auto all window roll down" (where you hold the "unlock" button down and all windows and sunroof slide open) only intermittently. Then it stopped working all together. My spare key seems to work right now, but I am concerned because my friend's keys did the same thing, and his spare stopped working about 3 weeks after his first key broke.

Anyways, the dealers know or should know about this by now. MB is apparently sending a bulletin around explaining it. Dealers will give you a new set of reprogrammed keys.

If you have a "Smart Key" car, I'd recommend that you check to be sure that ALL of its functions are working properly. And, if not, call your dealer to arrange for the upgraded keys (I would try to do that even if my keys were working, just to get the upgraded (and hopefully improved!)keys).

Its a pain when the key stops working. You can still get in the car with the metal key, but its a pain to lock all the doors and you can't set the alarm.