My 96 C280 w/143k miles got a CEL and a P0400 code. I replaced the EGR tube & EGR valve (following some advice and posts from others on this site) then cleared the code.

After about 10 miles the CEL came back on w/a P0400 and P0411 this time.

What am I missing here? What's next to test? Replace? Any likely culprits?

ANY help would be greatly appreciated. Thanks!

Matt