Nox is created by high combustion temperatues,usually caused by lean mixture.Replacing the Cat may get the vehicle to pass for now but you're not addressing the root cause.Try one of those BG intake system cleaners and make sure the hot air door in the air cleaner is closing the whole way as both of these could also cause high NOX.If the co% reading was way low chances are it's too lean,what HC reading did you have ? possibly a lean misfire is the cause of the stumble.