Engine operating temperature is often overlooked as a contributing factor to high NOX output. A ten degree increase in engine temperature will affect NOX output by a certain percentage. Duke2.6 may be able to shed more light on this particular area of combustion science. Suppose your radiator has low flow, on your temp gauge you see no real visible signs until you decide to pull a steep grade. When you begin your ascent you begin to notice the temperature increase and in most cases you may dismiss it because you are climbing a hill. The radiator may be flowing 70% to 80% of the volume it was designed to flow. Because it is so gradual over time as a driver you begin to unknowingly adapt to this gradual increase in operating temperature. In some cases a new t-stat, a new temp switch, or a new radiator (maybe a combo of any or all) will have more impact at reducing NOX than what is often considered.

This assumes that you have eliminated primary contributors and or emission components/devices that are responsible for either increasing or decreasing the overall NOX output. (Engine timing, EGR Valve & control, Catalyst, etc warrant that a technician may need to verify their correct operation before reaching a conclusion)