Check Engine Code P0420, O2 Sensor replaced?

[mybenje]

I have been searching the forums, and it looks like some of you have gone through this. My 04 4 Runner SE, has about 90K, I am in California, I realize I am over the 80 K emissions warranty. My code is Catalyst code bank1, I think that is driver side.
By reading trough the posts, it could be either O2 sensor or Catalytic converter. Since the later costs up to $1800 which is crazy,most likely it will be the O2 sensor, more commom to my understanding. I am willing to replace the O2 sensor first.
My question is which O2 sensor should I replace on Bank1, the one after the Catalytic converter or before? Is it easy to replace, does anyone have instructions? Also a good reputable to purchase O2 sensor for California would be greatly appreciated.

Thank you for your help,

 

[r32]

Does this PDF help?

Toyota TSB for Engine

 

[toto]

P0420 and P0430 are those strange codes about catalyst efficiency. The computer sometimes messes up the air/fuel mix intentionally just to see how the O2 sensors respond, which indirectly tests the catalytic converter. Odds are that the cat is not the problem, but instead it's one of the two sensors. Another possibility is a leak in the exhaust system.