1990 300sel, check engine light+story

[selpos]

The summer of last year ('06). The check engine light goes on in my car. I bring it to an independent car shop that specializes in fereign cars. They check it out and tell me that according to the ECU there was an air injection malfunction and and O2 sensor malfunction (codes 11 and 17). They test both and the O2 sensor doesn't respond. They retest the air injection control and conclude the MAS unit isn't turning the air injection on. I have them replace both the O2 sensor and the MAS unit. A couple days later after work one night, I notice the check engine light is back on. I return to them with my story and they check it out and say this time that the MAS unit is faulty and they'll order another one (apparently 20% of the MAS units for my car are returned). I have the MAS replaced and everything's going well but another few days passed and the light comes back on again. I return, this time for the last time, and they suggest installing 'additional circuit protection' for the MAS unit and relays for the air pump and O2 heater circuit "to protect MAS unit from intermittent short". Almost instantly, after having driven off the lot, the light's back on! At this point, I feel like I'd be wasting more time and money just going back to the same place and the frustration was just too much for me. By now, it's the end of '06.

In October of that year, I took a smog pretest. The mas your NO emission could pass by with was 757, mine was 960. Both my HC and CO emissions on the same test were average and just below average, respectively. I'm told by a friend of a friend who owns a model of the same car just a couple years older and who's a car buff and ex-GM employee, that it may be the 'Exaust Gas Recirculation' center (it directs super-heated NO emissions back to some filter, this is where my memory of our discussion gets fuzzy).

I'm at a loss. The point is: does anyone have any advice that differs from this friend of a friend's, or do you think he's got it right? And a faulty EGR is to blame? And if anything, does anyone know anyone in the northern California area who's a knowledgable Mercedes mechanic?

Any feedback would be very, very welcomed. Thanks so much for sitting through the story (in my mind, to have an informed opinion you must be informed completely of the situation).

-J

(my goal is to get it to pass smog at least)