This may sound odd, but have someone check the catalytic converter. Most of Utah doesn't have smog inspection. I was raised there and knew many people who had their CC gutted or bypassed at the first sign of a problem.

is there anyway to check the cat myself?

Not that I'm aware of. Any muffler shop could do it, though.

I had the same problem with my 1984 380SE but I have double the miles. I went with a after market cat, new muffler as the old one was the original one, and a new resonator as it too was the original one. I passed the smog test but my NO was pretty high, everything else was about half the medium.
Last weekend I had the air/gas mixture set by an Indy and he told me it had been running rich. Wonder if that is the cause of the higher NO?

Also the complete job was just $300.00 and change at a local muffler shop.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.