On this particular engine, there is a real well-known EGR problem. On the EGR valve, there is a pipe which channels the exhaust from the exhaust manifold (through the EGR valve) and around into the intake manifod. These pipes are just notorious for plugging up with soot. They can either be cleaned out or replaced, I prefer to replace this pipe, as the new pipes are modified. I would almost bet that this is the real problem. Are you taking the car to a Mercedes or at least European specialty shop?
PS The codes will point usually to the "general" problem, not exact. There is no code that says "EGR Valve is bad", the code will read something like "EGR system, incorrect flow" and it can be anything with the EGR system, such as the vacuum line that operates the EGR valve is disconnected (or the EGR pipe is plugged solid with soot
). Same thing say with an engine misfire. The code may say "Misfire Cyl. 1", so what do you do, replace #1 cylinder? No, you figure out the reason for the misfire. Hope this helps clear things up a little.