I'll try to make this short. Had my 1991 z28 (350) smogged twice. The first time was after I did not drive it for about two years. Once in a while I would go around the block. Took it in, failed bad:
%CO2 (15mph) = 12.3/ @ (25 mph) = 11.1
% 02 (15 mph) = 2.2/ @ (25 mph) = 1.3
HC (15 mph) = 220/ @ (25 mph) = 188
MAX @ 15 is 108 MAX @ 25 is 83
CO (15 mph) = 1.28/ @ (25 mph) = 4.77
MAX @ 15 is 0.71 MAX @ 25 is 0.58
NO (15 mph) = 156/@ (25 mph) = 68
MAX @ 15 is 767 MAX @ 25 is 706

After the test I changed the oil, oil filter, air filters, new tank of gas, throttle body cleaner, fuel injector cleaner (stp-jet fuel stuff), AC Delco spark plugs (from Chevy dealer), distributor cap (Chevy dealer), fuel filter (Chevy dealer), PCV (dealer)

I changed a vacuum line that was thrashed

I took it back:
%CO2 (15mph) = 12.9/ @ (25 mph) = 13.6
% 02 (15 mph) = 2.2/ @ (25 mph) = 1.7
HC (15 mph) = 152/ @ (25 mph) = 85
MAX @ 15 is 108 MAX @ 25 is 83
CO (15 mph) = 1.01/ @ (25 mph) = .42
MAX @ 15 is 0.71 MAX @ 25 is 0.58
NO (15 mph) = 285/@ (25 mph) = 125
MAX @ 15 is 767 MAX @ 25 is 706

So as you can see, the car did a little better. Any suggestions? The car runs really strong, oil pressure is great. When driving, I do not get any Service Engine Soon light. I did fail my "fuel cap functional." It is broken...not to the point that it doesn't pressurize, because it does. I can hear the pressure release when I remove it. I hope that I submitted enough information. Thank you for your help guys, I really do appreciate it!!!!

 

Failing HC is usually because there is an incomplete burn in the combustion chamber. The air fuel ratio is incorrect.

Failing CO is usually a lack of air entering the engine.

I would replace the O2 sensor, set timing to stock settings and clean the throttle body. Also, I would wait until all of the old fuel is used up and refill with a fresh tank of fuel.

 

DO you still have the original cat? Cat's die just like any other part... If all else failed (including the fresh gas), replace it.

 

There are three things that effect the no readings. The cat, egr, and combustion chamber temps. Run premium gas for the test. It wouldn't hurt to put in a can octane booster for the smog test. Get the chrome can by Nos. It is the only octane booster that does anything but take your money. It will cost you $11.00-$12.00 per can. Make sure your egr works. Sometimes the passages will plug up and the egr solenoid works, but no exhaust gas goes through. Finally, if you have an original cat, you may need a new one, but I would try to fix everything else first. When you go for the next test, have tech do a pretest first to see if it will pass. No need to keep telling Sacto that your failing over and over.

 

I'd suspect the O2 or CTS as a guess.
A bad cat would show on the nox #'s.
A bad egr valve will throw a code unless the O2 is bad too.
The code checks for an 02 swing to verify the egr valve opened.
Though a stickie one might cause a problem.
Also make sure the car is good and warm first prior to a test.
No sticky thermostats, and drive around a while first.
I'm only two blocks from my smog shop.....
I leave the car running while I check in if I suspect a problem.
Higher temps help a lot on the HC.

A dirty injector might cause this too, or you might just need to run it hard to get it cleaned out.

After you pass, expect a test only slip with your registration bill in two years.

If you really need too, I can scan it for you to see if something is off. I'm working night's and really busy right now so it's hard to get a hold of me.