Originally posted by Benzmac
There should be a thin black key that resembles the smart key, but is 1/4 the thickness. All cars with smart key came with them.
Your car should have come with two SmartKeys of "full size" and one "flat key" which is a smaller thin plastic key that has a removable small metal blade for opening the door and a transponder for starting the car.
A common failure with SmartKey cars seems to be the key itself, but my C230 had the EIS go bad. If the problem persists with a new key or your flat key, then you'll probably have to replace the EIS. Otherwise, the key is at fault.
Is the car under warranty?