Hi nox is also an indication of very lean fuel mixture. Do you feel a loss of power. I Replaced the ecu no change. For fun I disconnected the fuel pressure reg. and it passed inspection,also feeling a return of power that I had not realized I was missing. (5speed). The problem is back again? I had replaced the cat did not fix.The shop that replaced got me an inspection sticker. It failed 2 year later they replaced under warranty and the nox were even worse. That is when I disconnected the reg. and it fixed it.