Lately, the most common failure cause for high NOX, if everything checks out normally, is the aftermarket Cat-Converters. I'm especially refering to the small units that used to cost around $100 -$150 installed. These small units skimped on the Rhodium element that catalyzes the NOX and rarely will make it past the second smog test. The state of CA requires a new, "CARB EO" certifed converter to be installed now that the manufacturers have to warranty for 5 years. While working at the Jeep dealer, I used to see alot of '91-'95 4.0l Jeeps that failed the NOX by 100-200 PPM then pass when a new factory replacement cat was put on with only 100-200 PPM NOX total (Usually around 600 ppm NOX is the limit).