Way back when in the early emission control era I did a MSME at the U. of Wisconsin Engine Research Center, where I did emission related research, so I am very well versed at an engineering level on how emissions are produced and controlled. As with most research, theory can provide a direction, then it's a matter of running tests and analyzing the data to see if you are heading in the right direction.

Retarding the spark advance map is a sure fire way to reduce "engine out emissions" and keep the catalyst hot so it promotes maximum oxidation and reduction reactions.

California has identified my car (and probably all M103-engined models) as "high emitter profile", and I have to go to a "test only" station.

My 190E 2.6 was getting near the limits about ten years ago, but it wasn't until I found out about the R16/1 resistor on this forum that I was able to figure out how to significantly retard the spark advance map, and it sure works like a charm to reduce the numbers and get more margin, but even I was surprised at the NOx reduction, which was about 90 percent on my car.

Retarding the spark advance map as I recommend reduces peak flame front temperature so much that very little NOx is formed during combustion, which is why the number is so low, but since so much of the fuel's energy is being dumped out as high EGT, the car will barely get out of it's own way below about 2000 revs.

I'm somewhat surprised at your high O2 levels, on both tests, but I've had tests where I was certain that the O2 equipment was out of calibration and reading way too high. I get about the same numbers as you other than O2 which is 0.0 percent. If everything is working satisfactorily including the O2 sensor and catalyst, then the O2 reading should be at or near zero. At 84K miles I still have the original O2 sensor, and it appears to be performing satisfactorily. Based on the initial test's CO readings I think your O2 sensor was okay.

...glad you got your car successfully tested, but there are still some guys who don't believe that my recommendations will have much effect and recommend fuel additives, a new O2 sensor and catalyst. Anecdotal evidence here suggests than inexpensive replacement catalysts don't last very long, so replacing the original cat should only be done if there is clear evidence that it is no longer effective.

Duke