Hi Guys, first time posting with a cat problem. I recently have spent a lot of time working on my 2001 civic hx. I found that my check engine light has come on with the p0420 code. Problem is

1. I just replaced the catalytic converter (or what I think it is) about a month ago with a semi new duralast upstream sensor. I purchased this one: 2. I'm not sure where the catalytic converter is because it looks like there are 2 of them? One underneath the car and one right out of the exhaust manifold.

I plan on replacing a couple sensors before I spend more money on another cat. I've put too much money into this car already and need to save up because I'm moving out at the end of the summer. Can anyone help me?

Thanks