My valve stuck closed at around 65,000 miles, giving me the P0400. The part was $109 and labor of an hour cost $90 at a local garage, been fine for 45,000 miles.

O2 sensors are a common failure item, you WILL be replacing one or more some day, but a P0400 is in the EGR itself. At the top of this forum there is a list of all codes, you can look them up yourself (and with an Ultragauge or similar app, you can READ them yourself, too.)

I see you have learned that taking a Mercedes to a Chrysler dealer is a bad idea. That is a lesson you should not forget.