Bigben, I looked up code 25 for your car and..
code 25 for your car in the Diagnostic Module (DM) states the following: "Knock Sensors faulty"
So you need to start there. Are you sure it is code 25?
Maybe someone can tell you what to look for and where to find this sensor. Not sure how easy or hard to do it yourself as I never had to replace this sensor before, but I know it is a screw type sensor that screws to your engine block and has a plug that connects to a connector and monitors the knocking of your engine and sends info to the brain of your car so it will adjust fuel mixture etc.. I am sure the pros here can give you better explanation.. Usually this sensor doesn’t go bad that often, so make sure it is code 25 (count the LED flashes right).
By the way, the fact that you clear the code and then the check engine light coming back 2-3 days later mean you got a problem and you need to take care of it. The code coming back mainly tells you there is a fault that requires attention (fix) and clearing the code as you describe doesn't mean the problem went away. You need to locate a good Independent mechanic familiar with your car and see if he can diagnose this for you. Or go se the MB dealer… one thing for sure, the knock sensor is not very cheap and you better off buying it from fastlane here or any MB parts store as the dealer going to charge an arm and leg for it.
Do a search by clicking the above "search" button and type things like "Check Engine Light" etc and do a search and you'll find plenty of info on how to read codes and clear them etc.. You need to make sure that the 25 DTC is the only code (problem) you have... With some time and effort you can find out a lot by using the search capability above. Let me know what yoy find out. Good Luck...