What's the code? Your car should have an LED under the hood. Near the passenger side hinge is a black panel with a 3" diameter cap with a hex fitting on it. Just ahead and inboard of this cap is a raised ~1" cube box with a flip top. With the key in the on position and engine off, press the button for 2 seconds, then count the flashes. Press the button for another 2 seconds and count the next set of flashes. Repeat until you're back to the first set of flashes. The codes will be presented in ascending sequence so when you see the lowest number again, you've read all the codes.
To clear the codes (ONLY AFTER YOU'VE READ ALL THE CODES!), hold the button down for 9 seconds after the code is displayed. You have to clear the codes individually. Then remove the key (for at least 10 seconds?) and start the car. The check engine light should go out and stay out unless something is really wrong or has recurred.
Item 6 at http://pages.prodigy.net/jforgione/MB_S500.html
has a lists the fault associated with each code.