If the converter is original, I would bet it's empty. A good running car can still pass on HC and CO. The IM240 test that they've been using is nailing people left and right on NOx, and mostly it's due to bad converters. Vacuum leaks, lean mixtures (for other reasons) and bad combustion chamber deposits are among the things that might be causing problems. I'd still say, if you are driving on a 1985 cat (still 3-way but old) it is gone.
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1992 940 wagon, very low miles
as well as others.

You DO have a bad cat. However, since your car is pre-1989, you can adjust your timing. And setting that to 0° will bring the NOx WAY down.
The car will be down on power. The HC and CO will be lower as well.
It's a very easy 'fix' not recommended by the EPA. But you're in Canada eh!
Do what you gotta do!
That simple measure will typically cut the NOx in half.
Tuneup items will all help and really must be all ok to make sure you'er not getting unburnt fuel down to the cat.
Good luck with it.
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Rob Bareiss, New London, CT 86 244, 87 244, 88 744

See the FAQ. High NOX is often the result of a vacuum leak causing a lean mixture. Check the intake system carefully, especially the air hoses and gaskets.