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I'm not sure if plugs would make the engine run warmer. When you talk about "hot" and "cold" plugs it's usually a reference to how well they conduct heat away from center electrode, I think.
It could be your putting in a clean air filter. If your old air filter was pretty nasty then airflow would be reduced and possibly causing a rich condition which would account for your previous high HC count. A byproduct of a rich condition is normally cooler combustion temperatures which would tend to lower NOx readings.
Now that your engine can breath properly with the new air filter it is likely that it is running "leaner" than it was before and, unfortunately, a byproduct of running leaner will be higher combustion temperatures which will tend to elevate NOx readings. It's kind of a Catch-22, one goes down and the other goes up!
You can sometimes play with timing and/or mixture control and find a happy medium but if all else is in order maybe your cat is getting weak. If your HC and CO numbers are real low, and NOx is just a bit high, you might have enough lattitude to crank up the mixture and reduce your NOx to a level where it will pass.
You might want to check all vacuum lines to make sure none were dislodged when you performed your other maintenance or you might have some worn out vacuum lines. I know when I changed all of my vacuum lines about a month ago I had several that were just barely hanging on.
I'm sure that others will also have some valid suggestions as well. A friend of mine runs an inspection station here in TX and told me it's amazing what just coming in with a fresh oil change will do.
Good luck,
Justin B.
83 244 Turbo
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