smog issues

I have an 89 Jeep cherokee which did not pass smog so I changed the cat. This made very little difference. Before, all the numbers had low emmisions and passed with flying colors with the exception of the NOx, the NOx showed like 2870 at 15 and 2700 at 25 . once the cat was changed, the NOx then showed 3001 at 15 and 1485 at 25. I was told most likely it was the EGR valve needed to be replaced. I took it to a shop and he said it was functional, but the solinoid control by the computer was not being controled being control by the computer and that he needed about 3 more hours to diagnose the system to see why the computer was not sending signal to the solinoid. Im wondering is there a way for me to test the computer myself. Where is it located, and is there a way to check codes on it or something. also, could the O2 sensor have anything to do with the emmisions which are out of wake.? thank

NOx is due to low combustion chamber temps...what kinda thermostat are you using? Timing (too much advance) also contributes to excess NOx.

CO is the richness of the mixture. High HC is usually caused by spark plugs.

The cat does the rest. Always run the Jeep on the freeway for an half an hour before going in for the test.

Where do you live? I am in California.

im in california as well.
well the heater has not been warming up like it should, and when we took it for another diagnostic today, they said the engine temp was staying cool and they think it is from a stuck open thermostat. So we will see once they change that out and retest it. They also said that at the moment the O2 sensor seemed slow to start working and seemed it may be sluggish, but they said maybe that is caused from the engine not getting to the right temperature as well.
thanks for the input.

Yep..gotta get mine tested also...

Do the thermostat...get a stock one of course for smog.
Set the timing also...your chain might have stretched.