'01 Pontiac Grand Prix with a P0420 error

It has 98,000 miles on it. I changed the cat out with a magnaflow. After it was installed the service engine light came on with the p0420 error code again after about 50 miles. I changed out both 02 sensors and I'm still getting the same error code. I didn't have the code cleared but I disconnected the battery for 15 minutes and once reconnected the service engine remained off for about 50 miles and came back on. I'm not sure if that resets the computer or not. If it does then I'm not sure which steps to take from here. From what I've heard about this code, it points to an issue with the 3 things I've already replaced. Any ideas?

P.S. I have already checked for exhaust leaks.

The part you installed is not doing its job. O2 sensors will not cause this code. OE converter or a OE quailty converter will fix it.

One easy check is, put back the old cat ans see if it works, if it does not, you could have the wrong cat or assembled wrong or is jacked up.