What can I do to lower nitrogen oxide levels?

nitrogen oxide?nitrous system?

I think you mean nitrogen dioxide (NO2) not nitrogen oxide (N20) aka nitrous oxide.

Sounds like you failed an emmisions test. You'll probably need a new catalyc converter. That should clean up your emmisions.

yea wait do u mean NOx? for an emissions test... all NOx is is oxides of nitrogen which is caused by high cyl. temps...usually bad EGR, try usin higher octane gas and change your air filter...