It may be the wiring...but relays were installed when I replaced the MAS unit the last time. It was just a waste of time because the light still went on the next day. Of course that was to protect the unit from a short. I was told by a Mercedes Dealership that they wouldn't touch my car cause it looked as if the wiring had been tampered with leading to or involving emissions control (which is illegal). I don't know when this happened, most likely before I owned the car. I'd heard the same thing from these 'specialists'. Of course, I didn't hear anything about the wiring being 'tampered' with per say. They only refered to the wiring as a "big mess". Anyway, it's hard to tell where this problem started at or with whom it started (if it was anyone at all). So far the suggestion that my EGR valve/device has malfunctioned seems the most likely (from the smog tests) but then again, at this point that seems almost too good. Learning more about cars is on my 'to-do' list but right now I just need this car in working condition. Is there a definate signal that would point to wiring as the problem?