Don't know if you've solved the problem yet. I work at a Nissan dealer in NJ. Only California is harder to pass. They even fail us if the shift indicator is off a little. Anyway, try running some combustion cleaner through the engine. We use a product called BG, works great. I've personally had 3 vehs that failed for Nox. I ran a can of combustion cleaner through them & a can of fuel injection cleaner through & they all passed no problem. One of the replies talked about carbon build up. This stuff cleans out the inside of the intake, intake ports in the heads, & the combustion chabers. Including the valves. Just find a ported vacuum sorce(only sucks when throttle opened) & put a length of hose on it. Put the other end of the hose in the can a little at a time while keeping the revs up, about 2,000rpms should do, until the can is empty. Good luck.