'05 P0420 code. Whats wrong?

[torakki]

My '05 Aveo with 150K has the check engine light on (P0420). Originally, I thought due to the air intake boot had a split and sucking un-metered air. I replaced it, reset the light and it came back a few hundred miles later. My friend has a smog inspection shop so we hooked it up to his analyzer and all shows good. The code indicates either a cat, O2 sensor before and after the cat. The first O2 sensor reads a fluctuating number which is expected. The second one, after the cat reads a very steady value which is also expected. Sniffing the exhaust shows very clean values. The only number that was a little high was "oxygen" at idle. Showing 1.0 to 3.0 but at testing speed shows 0.0. What is causing the check engine light to come on?
Thanks for any suggestions...

[petrified.rabbit]

From my experience its probably the 2nd o2 sensor, post cat.

[torakki]

Yea, my smog friend hinted at that one too. One thing I forgot to mention, when we reset the light the first time. The smog machine has to have all the sensors reading to begin the test. I went back 3 times with the machine still waiting for the "Cat Sensor" not ready, to show "ready". a few days later, the light comes back on but the cat was ready.
Thanks for the response....