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Here's the deal, mein Liebchen. Your CO and HC numbers are far from normal, though passing, indicate RICH mixtures, while you know the excessive NOx can only mean excessive heat, associated with LEAN mixtures.
All the advice is right on, but speaks to generalities and not the typical 90's Volvo with LH2.4. That O2 you have sees all four cylinders and the computer it is connected to adjusts the mixture to all four as if they were one unit. A 90's 240 doesn't run in tandem 15 years after it was born unless you do some individualized care of the four combustion chambers.
Start with the most likely and progress from there: Replace the intake manifold gasket. (The vacuum leak advice should have been marked in bold) Check the exhaust gaskets. Do a compression test and OMG tell us it isn't 200+ anywhere. Oh, and did someone mention plugs? Pull them and scratch tick marks into the wrench flats to ID each according to its cylinder number. Shuffle them on the table with your eyes closed and see if you can match them up the way they came out. Yes, they all should look the same. All it takes is one lean cylinder to mess it up for the group.
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