I've been meaning to post this for a few weeks, so here it goes.
Once again, I used Bruce Young's trick with plugging the vacuuum hose
to the ICU. The NOx reading dropped by half, well below benchmark.
Thanks again Bruce. See below...

High NOx and Volvo/Chrysler Ignition

You have the Volvo/Chrysler ignition used on 240s from mid '81 thru 1988. It's been found that the V/C Ignition can contribute to High NOx, and that plugging the vacuum line to the ICU can reduce NOx levels, 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.

For a technical description, see my original post here. and a success story here.

Other positive responses are posted in this thread and this one.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

The Ignition Control Unit is below the washer reservoir on the right side. The plastic vacuum line ends in a rubber elbow that plugs into a round metal object at the lower front of the ICU.
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-Cool Volvo- 1982 240 4 Spd OD w/ 248K, 1989 240 5 Spd w/ 241K. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245