NOX is formed when combustion occurs in an oxygen rich atmosphere. This allows a nearly instantaneous burn, which creates a high temperature. The NOX is formed when oxygen combines with nitrogen, which happens readily at high temperatures. If the mixture is a little richer, the fuel will combine with the oxygen (combustion) a little more slowly. This slower burn will result in a lower temperature, which, in conjunction with less excess oxygen available will result in lower NOX. Incidental to this will be a drop in CO2, but CO and HCs will increase.

You may well have a vaccuum leak somewhere. Favorite places are any vaccuum tubing, intake manifold and fittings, and injector seals. The vaccuum leak will allow excess air to enter the combustion process.