check engine light stays on all the time

It is unfortunate that you are experiencing such an annoying event. The check engine light could be caused by any number of things, for something as simple as bad fuel or worn spark plugs and cables (if yours is gasoline) to head gasket. HAVE IT CHECKED! An authorized service professional who can plug into the car, read the codes and share what is wrong. If you have had the scheduled service as required by Mercedes Benz, it could be very minor and very inexpensive, if ignored, the changes of it becoming a major issue is guaranteed, equaling higher repair cost and higher fuel consumption. I have learned the hard way, those lights come on for a reason. Have your car checked. As yourself, when is it a convenient time to be stranded.