Nox is caused by high combustion chamber temperature. Some cars have an EGR (exhaust gas recirculation) system to overcome it, some rely only on the cat. Many Hondas use an EGR valve, so check and make sure it is not plugged or disconnected. You can see if it works by pulling a vacuum on the control input while the engine is idling. If the engine suddenly stumbles or stalls when you do, it works. If the engine smokes a bit it tends to plug up the EGR pretty quickly.
If the egr works or isn't there you will have to replace the cat. Be aware that its life may be short if you are running oil through it, but you were selling it anyway, right?
