I passed inspection last year after replacing my 5 year old convertor with a new one from FCP Groton.

The car barely registered any NOx, it was at 75.

I just failed today on the Texas two speed test.

The readings were:

High Speed (25 mph) Standard: 1163 Current Reading: 2207
Low Speed (15 mph) Standard: 1273 Current Reading: 3334

As you can see the readings are well above the limit.

2 weeks ago I got in the car and it wouldn't start right away. When it did start, it idled roughly. I checked the ingition wires and the wire to the AMM. They were all in place and okay.

I replaced the AMM and the car started and ran nicely as it usually does.

I will tear into the car this weekend, however, I was wondering if anyone out there has successfully come back from such a high NOx reading lately, if you would share your solutions with me.

Thanks for your help.
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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 272,067 miles looking forward to 300K badge (or sticker??). >>You haven't really worked on a car until you draw blood<< :-}

look for a vacuum leak.
look for a vacuum leak post combustion.
check your o2 sensor and make sure it is works as per spec. a multimeter on the signal wire at the firewall after the sensor is warm to ascertain it moves wildly between .4 and 1 volts give or take. a lazy, sluggish or dead sensor needs replacing.
check all connections for tightness/leaks: header to manifold, manifold to head, header to cat

look for vacuum leaks first.