Here's something I copied from the Internet a long time ago. It may help. Does your car use oil? Let us know what happens...
"Your catalytic converter(s) are clogged- probably the small ones in the front pipe. This happens in a 300E from oil consumption over a long period of time. A quick cure (other than the obvious one of buying a new front pipe for $1,200 or so) is to take the pipe off the car, and have a shop glass-bead the front (engine) side of the mini-catalysts a couple of
millimeters. The clogging seldom goes deeper. Proper performance is restored without damaging the catalytic effects."