You want your engine temp to be on the cool side of specification - but you want the
cat to be hot (it's the heat in the cat that rips apart the nitrogen molecules from the oxygen molecules and thus reduces NOx out of the tailpipe).
High engine temp - specifically, high combustion chamber temp, will raise the NOx levels. Your cooling system needs to be in good shape. Having your car tested on
a cool day helps a bit, too.
Does your engine "ping" while accelerating uphill?? Could be due to excess carbon
buildup in the combustion chambers and piston tops (carbon deposits increases the
compression ratio, and combustion temps). Every now & then, I put a tablespoon of
Seafoam into each cylinder, through each spark plug hole (warm engine; using a very long, thin, funnel which I made myself) and let sit overnight. Drive normally the next
day. Do not be tempted to add more and a tablespoon per cylinder as you'll risk
hydrolocking the engine (= real bad). Do this 3-4 times, every other day, for a week.
After a week (maybe, two weeks), the carbon will soften and the pinging will very slowly
go away.
Some folks have had luck pouring Seafoam into the engine while running. I have done
this sometimes but it always makes me nervous to pour in the stuff and listen to my
engine cough, stumble and sputter. I worry about what all that commotion does to the
rod bearings.