Smog test failed- high NOX

I smooged my truck in NorCal and it failed due to high NOX. I had a new cat welded on and retested, failed again due to high NOX. NOX level didnt change much by replacing the cat. Any ideas?? Thanks

my high nox was due to fouled spark plug do a quick tune up plugs cap rotor air filter and youll probly pass

also try the O2 sensor that will solve it potentially as well as the tune up