I didn't see this listed in your list of repair work, however, a few years back, my 1990 failed due to high NOx and I discovered that the thermostat in the air filter housing box had failed in the open position, letting hot air in.

I replaced the thermostat, and my car passed the next time it was inspected.

I hope this helps, and good luck with your next inspection.

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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 305,600 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}