The 3 things that will raise Nox are, 1,an overheating engine, 2, an EGR system that is not working, 3, a dead Cat....I can tell ya from experiance that the egr system is the biggest culprit but with that being said the second biggest culprit are those cheapy aftermarket cats most will only get you by for 2 to 3 years...Also advanced ignition timing will raise Nox