P0420 Code

I've been getting this code for some time and I thought I'd look deeper into it. From what I've read it has to do with the cat running below threshold values.

I've also read that it can be caused by bad spark plugs, leaking exhaust, funky O2 sensors or a bad cat.

I'm trying to go with the cheapest fix here.......Can someone clarify the P0420 code for me. Bank 1 - this is the O2 sensor closest to the engine before the pre cat? Also , what side is this left or right - no, let's use driver/passenger - too easy to mess that one up.

If anyone has had experience or fixes for the P0420 code, I'm all ears.....