Not sure if NOX goes down, nor how accurate this graph is:
but NOX seems to go up when leaner than 14.7 and doesn't return to original levels until about 17.3:1, at which point hydrocarbons start rising. My local epa testing station used to offer a test anytime for $20, though I don't know if it would red flag your vehicle if it failed or use the obdII port or what. You might want to call them, without saying anything too self-incriminating of course.
