If your engine is running properly the cat should last the life of the car.
People clog them up prematurely with rich running malfunctioning engine.
Any time you get a rotton egg smell from the exhaust you should do something about it.
I like to run a fuel injector cleaner twice a year. It keeps the cat and injectors clean.
If you primary 02 is original seriously think about changing it.

is there a way to tell if youre running rich or not? it seems like i get pretty decent gas mileage.. what else would cause the high nox readings? i replaced my fuel filter, air filter, and put new oil in.

Possibly a bad fuel pressure regulator. I'd say go with the o2 sensor -it'* cheaper, but you test the fuel pressure regulator pretty easy.
If you don't know what it is, but see a canister mounted near the throttle body, that has a vacuum line going into it, if you disconnect the vacuum line and there is gas in it, then the fuel pressure regulator is bad.
For the o2 sensor testing, nothing other than a scan of codes, or replacement.

oops i meant to say EGR instead of pcv. i was wondering if there'* a way to test the EGR to see if its faulty, i know my pcv valve is good. i am concerned with the egr because apparently its sole function is to reduce nox. any way to test it on a 96 bonneville? it has an electrical connector with 5 or 6 pins coming out. does anyone have the pinout of this so i can hook it directly to a battery and see if it does anything? i am probably going to just go and get a new o2 sensor because i read water could damage it.. and when i replaced my upper intake, there was plenty of water vapor coming out the tailpipe, so i fear i may have damaged it by that. but i just dont think that a faulty o2 sensor would cause nox readings 10 times as high as they should be.. has anyone been able to test their EGR? thanks guys.

also.. i did not see any sort of canister by the throttle body. is there any other way to test the fuel pressure regulator?

Oops. I was thinking on a Series II non SC'ed 3800 the fpr was near the throttle body. I can't say I remember where it is exactly, off hand, but it is on the fuel rail. I had said near the TB, because that'* where it is on my SSEi.

found the big canister. its below the airbox and directly underneath the big electrical square thing next to it, i dont know what it is but it says delco electronics and its a long and narrow rectangle shaped thing. but anyways.. there were 3 hoses coming out of it.. one large one and two smaller ones.. pulled both smaller ones off and they were both completely dry with no gas smell to them. the smallest hose, which looked like a vacuum line, had a pressure relief valve on it and i was able to trace it back up to the engine. it had some connectors that went into the throttle body. i have recently been having trouble with these lines, sometimes when i start the car one of them will pop off and i have to open the hood and put it back on or the car will run rough. i replaced all the rubber on them, so they should be solid now. so i guess this rules out the fuel pressure regulator? i am still convinced that it is the egr and not so much the o2 sensor, because the only thing i was over the average for in the smog test was the nox. any way to test the egr?? does anyone have a pinout of it? thanks.

I would expect a bad EGR to give you a service engine soon light... NOx is a sign that your engine is running too lean and too hot. This can happen without enough fuel, too. If your charcoal canister is becoming disconnected from the engine, it might cause it to run lean, but the oxygen sensor should compensate.

just finished testing the oxygen sensor. i read that the bias voltage when the sensor is disconnected should be around .45 volts (measuring the purple wire to ground on the 4 plug connector with the o2 sensor disconnected). i measured it at 0.9 volts. seemed a little high? i then put a wire touching the purple lead inside the connector, and reconnected the o2 sensor. i measured the voltage of this wire to ground, and it fluctuated between around 0.45 and negative 0.45 volts. so can anyone shed any light on whats going on here?

got a new o2 sensor and one of those fram airhog air filters. now i get about 30 miles to the gallon, but im still getting a check engine light and my car is still running nox about 10 times what it should be. i got the scan tool from autozone and im getting codes P1441 and P1870. can anyone help me out with the codes, i really dont know where to start. thanks.