Read this article by Steve Brotherton -

http://www.peachparts.com/Wikka/EngineControls

Actually, skip to the last couple of paragraphs. Steve describes a problem with the M104 engine wherein the PCV system deposits soot in the intake manifold and that somehow causes an EGR fault. I don't have an M104 so I didn't read it carefully.

Sixto
95 S420
87 300SDL