P0420 fault source

My 2005 Prius with 292k just started putting out the P0420 code. just last week, I replaced the timing chain due to a bit of chain slap noise, and my mileage has gone down over the last year. Runs much better now, but just before I changed the chain, the P0420 showed up, I was hoping it was linked to the bad chain, but no luck. Here is what I have recorded for troubleshooting information. Running at 65 mph, I used Live Data on my scanner to log the upstream O2 ( Air fuel ratio) , the cat temp, and the downstream O2, the upstream stays pretty steady at 3,190 to 3.339 , no matter if I floor it to pass a car, and only really changes when I let off the gas and the engine coasts, then it briefly goes to 4.997 before dropping to 3.3 The downstream jumps all over , from ,926 to ,315, when coasting, it drops to ,015, then resumes the erratic jumping while at highway speed. the cat temp is 1235 to 1364 on the front part, and 1065 to 1170 on the back one The tutorials I have viewed say the downstream should be steady, and my cat temps look pretty normal, with the mileage, am I looking at a cat replacement, or one of the sensors ? the car goes through about 1.5 quarts or so every 5000 miles, my oil change schedule, and I have seen that oil can kill the cat, but it has been doing this since I got it a couple years ago, and the P0420 just showed up a couple weeks ago. Any advice on where to start ?

 

I was hoping that was not the one going, it is the really hard one to get to, but I was afraid that the A/F signal barely changing under widely varying loads was a bad sign, I ordered a set of O2 sensor sockets, and have a Denso upstream sensor ordered, I see on other threads that they recommend staying with factory replacement parts. Probably won't get to it until next weekend with my work schedule, I will let you know how it goes. Thank you for your quick reply.