To the original poster, I am sorry I missed the part about "high NOX" in your original post. my bad... I would listen to Nixspeed though, it makes total sense.

In the link I posted, I remember typing that I also had to adjust my timing just right to pass along with the cat.

Thanks Nixspeed for clearing that up.

 

No problem. Emissions are tricky and I'm no pro, just had a few cars I've diagnosed and fixed. Everything has to be in balance.

 

Had same problem with high NoX here in CA. Problem was with EGR system even though EGR valve itself was good. Had vacuum controller replaced and all passages thoroughly cleaned (they were full of carbon buildup). Passed with flying colors.