How can I lower my High Speed Nox (ppm)?

Check EGR, and Carbon build-up

ck egr passages for blockage of carbon or timing can be retarded or poss o2 sensor

Globalhelpers suggestion first!! EGR SYSTEM function is very important. Next would be timing as he mentioned , and then be sure the cooling system is in good shape. Last , if high miles (by age I'm assuming it is) maybe a top engine cleaner -get carbon build-up off pistons -lower comp. lower combustion temp=lower Nox.