'05 P0420 code Low voltage on downstream O2 sensor?

[torakki]

Tomorrow, I'll check the manufacture of the 2 sensors I got. I believe there are the same brand I got in the past and possible there was nothing wrong with them at this point. I know they weren't $225. My friend at the smog shop says that he's had GM cars in the past that throw that code, as if, the factory programs a very narrow margin of error. Since the car has clean exhaust from the tail pipe.
Updates soon, I hope....
Thanks for the response