You have the Volvo/Chrysler ignition used on 240s from mid '81thru 1988. It's been said that the V/C ICU's design can cause hi NOx, and that plugging the vacuum line to the ICU can lower NOx production with little effect on normal operation. Note that this is not a guaranteed fix, but worth a try if your other numbers (and cat) are good.

I first posted it (with technical details) on 5/21/04 (see original post here. ) At least one person has found that this solved his high NOX problem.

You can see his happy response at success story.
NOTE that he had failed with a new cat, then plugged the ICU vacuum line, put the old cat back on and passed!.

Another success was recently reported on this thread.

And yet another one here.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.