P0136 Code Help!

Got a 2001 Tacoma Prerunner V6 auto...154K

I got a p0420 earlier in the year so I got a URD o2 sensor sim for that problem. Ran great for about two months.

Then this code popped up...the p0136! Upon researching and reading, I think it might be the rear o2 again, correct me if wrong.

Do I need to replace the rear o2?

I though that buying the o2 simulator, it would solve this problem. What can it be?

If I have to buy a whole new sensor, did I waste $$$ on the o2 sim?

NEED HELP!!!

PS...I've been getting SHITTY-ASS gas mileage as well...best was 16.5!!!...with a Prerunner!!!

thanks.