'05 P0420 code Low voltage on downstream O2 sensor?

I've had the engine light on for a while and knew it was the P0420 code. Now, I'm fixing it and it was suggested by my local mechanic, it was probably the downstream sensor. So, I replaced it. Light came back on in 2 days. So, I replaced the upstream sensor, light came back on in 2 days. My friend, at his smog shop, diagnosed the down stream sensor was showing a Low voltage reading. Something like .1 instead of .7 or .8. He said it indicates oxygen getting into the flow, like a leak. No leaks found. The cat is the only thing left? Or is it? Car has 176K on the dial so maybe the cat....
Thanks for any suggestions.

Not sure if this would be related or not but I just went through a fiasco with an O2 sensor that needed replacing.

Just fyi.

Tried 2 different after market Chinese crap sensors, one lasted a day before it triggered a code, the next one lasted two days before it went.

We then went to a GM OE sensor, the difference being $225 !! and it's been fine.

These cars appear to be notorious for burning thru sensors early and are just a notorious for liking the expensive OE sensors as a replacement.

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

The sensors are NTK (NGK) out of Michigan. Good rep in my opinion. In the meantime, the key broke off in the ignition so the Cat, engine light issue is on the back burner for a few days.
Thanks for the input and Merry something or other.....

I just wanted to follow up on the engine light issue. I went back to my friends smog shop and we ran the car on the dyno (rollers). Both o2 sensors were doing exactly what is expected for a good running engine (per the numbers). So, the cat is good and doing it's job. Emissions out the pipe were clean. Should pass (except for the code). So, we deleted the code and I drove my wife to work and came back to the smog shop, driving smooth the whole way. All monitors were reset but one (which is OK). Smogged it real quick and I'm good. I expect the light to come back on in a day or so, but for the next two years, I'm the winner.
Hope this helps someone in the future.....