There will more than likely not be an inexpensive fix for your Nissan 300zx's high NO problem. Most often the 300zx failed the smog check for high NO due to a faulty EGR valve and EGR valve system; the EGR valve gets plugged up with carbon and needs to be removed in order to be cleaned out. The problem is removing the EGR valve requires the removal of the intake manifold, disconnecting the engine from the transmission, and pulling the engine forward in order to gain access to the EGR valve, EGR hoses, and ports. I've just described the wort case scenario for fixing a high NO failure on a Nissan 300zx.
If your high NO problem is not due to a defective EGR valve, and maybe because of a lean fuel condition, vacuum leak, or defective fuel feedback system (lean condition because of emission sensor failure... faulty oxygen sensor or mass air flow sensor), then the repair could be significantly cheaper.
You should consider having a smog check repair station conduct a hands on diagnosis in order to determine the true cause of your Nissan 300zx's high NO problem and take it from there. Hopefully the problem is not the EGR valve.
