I had the same high NOx problem with my 91 CRX Si. I fixed it by doing a complete tune-up (new plugs [remember to gap them], new air filter, set the valve clearances, set the timing, change the oil and oil filter, change the fuel filter [if it's over 5yrs old]). That brought the NOx enough to pass. Before the next smog test, I'll probably replace the cat. [it brings down HC, CO, and NOx.]

If your car has EGR (exhaust gas recirculation), cleanup or replace the EGR valve. (that should bring down the NOx; NOx is caused by high temps in the cylinders; the EGR lowers the temps.)

How many miles does your car have on it?
Does it still have the original cat? (at about 150k the cat starts getting weak)

You can by a cat for $107 including shipping. (it could be more, if the cat is bolted directly to the header.)

http://www.catalyticonverters.com/ho...converter.html
http://www.cheapexhaust.com/honda-civic-converter.html