Got a 2015 dodge journey2.4L 4 cylinder, had been driving it around with this code on for a few months, just couldn’t get it checked out earlier. came to the conclusion I needed a new Catalytic converter so I got one and installed it myself. Have been driving around for about a month or 2 now and i keep getting the same P0420 code. I have a FiXD reader that gives me live data and the 2nd O2 sensor (Post cat) the reading isn’t steady. it fluctuates from like 0 to .7 volts.

just looking for any advice on how to fix this issue, i’ve cleared the code at least 7 times by now but something must still be wrong. any help would be appreciated