Do you mean NOx?
NOx forms when fuel is burned at high temperatures, as in a combustion engine. The primary sources of NOx are vehicles like your truck!
NOx causes a wide variety of health and environmental damage due to the various compounds and derivatives in the family of nitrogen oxides, including nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide.
NOx kills you, me and the planet. So if you have failed your local emmisions (smog) test, you may want to look into replacing your emissions (smog) system. Start with the smog pump and catalytic converter, have them inspected and then move on from there.
If you require more assistance. please visit http://www.FordTruckClub.net and any of the knowledgeable members there can provide you with plenty of helpful advice to fix your issue.