Go Away, P0420 Code!! What could it be?

I've had a P0420 code for quite a while on my '05 Aveo. There is "nothing" wrong with the car. My friend has a smog shop and have run it on his machine a few times. It burns clean and would pass smog, if not for that code. I've replace both, up and down stream O2 sensors. My friend says, the cat is working fine, from what he reads from both O2 sensors. He thinks it may be a fluke of some kind with this car. I have 188k miles on it and runs like new. Does anyone else have this issue? What could cause it? Thanks for any suggestions.

Your friend is wrong. You can continue to search for another answer if that's your choice, but sooner or later it will come down to replacing the cat because it is no longer functioning properly.

Well, maybe it's time to look for a new cat. I didn't really want to search for another answer, just the right answer.
Thanks for the response.....