Smog problems with 94 suburban

Well here's the story, I have a 1994 Suburban 2500 with a 454ci in it. The truck runs good with 165000 miles on it, and the last time it passed smog was 4 years ago. I had to put a new cat converter on it back then but had no other problems with it. Well two years later it was time to smog again, and guess what, FAIL! For high nox levels. We have only put maybe 10,000 miles on it since the new cat was put on yet the guy told me it's either the egr valve or the cat. I go home and decide to put it on non op until I have some extra money to get it taken care of. Well now is that time, I replaced the egr valve, did a tune up on it (plugs, wires, cap, roter, air filter) and confidently take it in to go smog, but again it fails for high nox levels, but not as bad as tge last time. This guy told me it's probably the cat and I told him it only had 10000 miles on it but he told me just sitting for a long time can ruin the cat as well. The truck had sit for about a year but I was hesitant to beileve him.
At this point I'm not sure where to go, I really don't want to spend 400 on a new cat if that's not the problem, but at the same time I don't know what else could be the problem. If anyone here has any experience with these motors and smog related stuff I would greatly appreciate some input! I'm pretty mechanicaly inclined, but when it comes to smog equipment im lost! Haha thanks for any help!

Welcome to AF.

Check for vac leaks especially around the intake and throttle body base. Check all vac hoses for cracks and/or leaks. Replace the PCV valve.

Well here's the story, I have a 1994 Suburban 2500 with a 454ci in it. The truck runs good with 165000 miles on it, and the last time it passed smog was 4 years ago. I had to put a new cat converter on it back then but had no other problems with it. Well two years later it was time to smog again, and guess what, FAIL! For high nox levels. We have only put maybe 10,000 miles on it since the new cat was put on yet the guy told me it's either the egr valve or the cat. I go home and decide to put it on non op until I have some extra money to get it taken care of. Well now is that time, I replaced the egr valve, did a tune up on it (plugs, wires, cap, roter, air filter) and confidently take it in to go smog, but again it fails for high nox levels, but not as bad as tge last time. This guy told me it's probably the cat and I told him it only had 10000 miles on it but he told me just sitting for a long time can ruin the cat as well. The truck had sit for about a year but I was hesitant to beileve him.
At this point I'm not sure where to go, I really don't want to spend 400 on a new cat if that's not the problem, but at the same time I don't know what else could be the problem. If anyone here has any experience with these motors and smog related stuff I would greatly appreciate some input! I'm pretty mechanicaly inclined, but when it comes to smog equipment im lost! Haha thanks for any help!

with the EGR operating correctly this should be the fix...replacing with a new EGR with defective control lines ,wires, solenoids, as well as the passage ways that the controlled gasses flow may be the cause ...the guy was correct it is 95% of the time the EGR not operating correctly..

the cat converter does not reduce NOX ...the cat converter controls hydrocarbons which is unburned gases....


vacuum leaks can very well screw up the EGR function...