'05 P0420 code. Whats wrong?

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...

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

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....