We're going have to look at the CO results as well as the NO results. It is
possible the high NO is a result of a lean fuel condition. If in fact the
EGR system is working; the EGR valve opens as required, and when it does
open, it sends exhaust gases back to the intake manifold then you have to
ensure there is no lean fuel condition.
Try opening the valve manually during idle. The idle speed should drop
almost to the point of stalling the engine. This indicates the EGR valve is
working. You then have to make sure the solenoid is receiving power during
off idle when your Seville is being driven. If that too is working, then
you'll need to make sure the oxygen sensor is sending a signal to the
computer and the computer is adjusting fuel flow.
