high nox readings

need some help with this 92 lumina 3.1 van. it failed emission test for nox readings too high. no check engine lites are on and no stored codes. where and how do i start?

Egr system operation and or clogged passages number one issue, engine running to hot number 2, bad cat number 3.

In my experience, hi NOx levels usually point to one of two things: a malfunctioning EGR vavle - reduces NOx by introducing exhaust gas into the combustion chamber and reducing operating temperatures;

or your catalytic converter is too old and is no longer completing the combustion process - catalyst converts most of the remaining NOx to CO2.


edit: ah, ya beat me to it... :icon16: (yeah, what maxwedge said...)