OBDII catalytic converter code? LSD whining noise? ST sway bar endlink?

[az_240]

Was just cruisin along when my cel comes on so I go to autozone and they tell me I have a P0420 code which is the 3 way catalyst system and they say they can not erase the code for some reason.

I plan on getting a turbo kit for my KA when I have money so I figure I might as well just get a 3" high flow cat or resonated test pipe instead of buying a stock replacement.

Which high flow cat can I use to make the code go away? Do I need a special 3 way cat? Is there a 02 sensor inside the cat or something? I dont really understand how a cat can just go bad and throw a code like this.

This is the cat right below the exhaust mani correct?

Also I installed a cusco 2 way LSD, spl subframe spacers and ST rear sway bar and now I get a slight whining noise (mostly during acceleration). It gets kinda loud when im on the freeway going about 80.
Is this normal or do I need to move the flanges on the diff around? It doesnt clunk when going around corners but it does feel pretty rough only when I turn right and go over bumps while turning... Is that normal?

1 more thing... the endlinks I installed with the suspension techniques rear sway bar can not be tightened very tight without the poly bushings getting smashed and distorted which is how im driving on them now. Is it ok that they are smashed or should I back off a lil bit? I didnt want the endlinks to be too loose or theyd just fall off. Id get the spl rear endlinks but they want 200 bucks!

Any help is greatly appreciated.