G20 NO too high

First and cheapest is to get a high-quality injector cleaner and run the entire tank of gas with it.

Second, and second cheapest, replace the sparkplugs and air-filter.

Worked like a charm for me just two months ago when my minivan failed.

Well I used to have a california smog license but quit. Nox is usually fixed by making sure EGR valve is working properly. I am not that familiar with the Nissan system but you could look at vacuum sticker on the hood and also emission sticker to see if EGR is listed. Some cars don't have EGR. If you don't have an EGR then the catalytic converter takes care of Nox. You should make sure the converter is being fed a proper mixture of richness (more gasoline). If you are running lean (any left over oxygen in smog report, and super low Carbon Monoxide means you could be running too lean so reducing part of catalytic converter might not be getting enough CO (carbon monoxide to work) sometimes changing a lazy or miscalibrated oxygen sensor will pass Nox. If not then maybe changing the catalytic converter might help. Also check timing as over advanced timing increases Nox but usually less than 100 pts. If pinging this might also generate Nox so for the test you could run a high octane gas. Well hope this helps.

The cheap gas you're using. Low octane fuel is the major cause of high NOx readings. This is a high performance engine and using 87 octane gas is a bad idea.