Help!!!!! code po421

[03DMeclipse]

hey guys need a little help with this one...

heres a little background...when i installed my magnaflow cat-back and kamikaze header, i had to purchase a universal o2 sensor for the header. well that one did work so i replaced it with a factory o2 sensor. i had the code cleared..

Now i got this code, p0421 for the rear o2. the only thing i can think of why this came up and i accidently unbolted one of the cat-converter and it hung from the other boltso therefore it might really be damage.

My question is, what are my options of fixing this..purchase a new one, buy an 02 simulator or relocate it behind the main cat(i know what that will do)...i have no clue on what to do????