Just wanted to share some info (1st person experience) about what a Catalytic Converter will do, to help someone solve an emission testing problem.

Car: 1993 B230-L engine, automatic transmission
Test: NJ State Inpection Station running the on-a-dynomometer (at road speed) kind of test.

Past history:
Gas: Max - 2004- 2006- 2008
NOx: 970 - 190 - 964 - 2021
HC : 128 -- 028 - 034 - 085
CO : .71 -- .09 -- .15 -- .44
CO2: ------- 015 - 016 - 014
O2 : --------- 0.2 - 0.0 - 0.6

Clearly, in 2008 (just last week), the car failed for NOx (and had just barely passed the time before, in 2006) -- it had been gradually failing over these years.

I took it to my local Volvo dealer with the instruction to change my Cat (based on my years of experience). They checked it and confirmed that it looked bad, and although they couldn't test and pass the car themselves, lacking a dyno, they said they'll do their best t get it ready for a retest. (Note: I don't like the work of some local indy shops, even though certain ones are authorized and equipped to do emissions repairs because they have, in my experience with them over the past more than a couple of decades, tend to use cheap ("Scantech") parts despite my telling them to use Volvo brand parts -- so I've taken to just going to the dealer and telling them what to do).
Anyway, the dealer changed the Cat and the oxygen sensor which (in these last model 240s) is plugged into the Cat rather than in the exhaust manifold, like in my '84.

Good news is that it worked. I took it back to the inspection station, and here are the new numbers:

Gas: Max - 2008 (with new Cat and O2 sensor)
NOx: 970 - 138
HC : 128 - 022
CO : .71 - .03
CO2: ----- 015
O2 : ----- 0.2

NOx, HC, and CO were all reduced. And hopefully these new parts will give me a few more years before replacement is necessary again -- I'm in the 2004 ballpark again.

Of course, I didn't have to take it to the dealer for this work -- I usually do repairs myself, but since I don't have a lift in my garage, and given my age (about to retire) and bad back, I didn't relish working under the car (albeit with jackstands) -- my wife insisted that I "save my back" and spend the extra money!

Well anyway, if you've failed for high NOx, consider this as a possible solution (assuming the car is running well -- otherwise, you'll have a new Cat "for a day" as Rule 308 likes to put it -- i.e., you'll ruin your new Cat in short order, and have to buy a new one again for the next inspection.)