I had the exact same issue and I just solved it today. All my numbers were normal except for Nox. I did a complete tune up and changed the oil. still failed. I did a carbon. cleaning with seafoam and changed air filter-failed. I checked again and again for vacuum leaks and found none. I made sure all my vacuum hoses were not clogged. I checked All my sensors. I Replaced my O2 sensor.- failed. I checked my timing and I even tried different emissions stations.finally I bit the bullet and bought a Cat. I then buried the cat up to its neck in the yard and ran over it with the Lawn mower. Now I feel much better.
-then I decided to install a new Catalytic converter from Advance
I cut the old one out and to my surprise there was NOTHING
I will be testing again tomorrow, but I am perty sure its gonna pass. I will update as soon as i get results.