Just for info - my daughter's Mazda failed on high NOx, and here is what I did to get it to pass:

By the way, if your HC and CO are low then your cat is okay.

High NOx is caused by high combustion temperature. This is caused usually by too lean a mixture or retarded ignition timing.

Lean mixture can be caused by:

1. low fuel pressure, so I changed the fuel filter
2. vacuum leaks, so I checked but found none
3. stuck open idle air bypass, so I cleaned and the idle slowed way down when the engine got warmed up
4. failed O2 sensor so I replaced that

Retarded ignition can be caused by

5. timing off - checked it
5. corroded dist cap terminals/rotor, so I replaced them

Car easily passed NOx on next try.