Ok so I had a problem with my oem cat, the car was driving very slowly as if it didn't want to go. The p0420 code came up and went away after a while

I took it to a garage and they showed me that I had a dent in my oem cat. I had it replaced with a magnaflow direct fit cat from car sound.

The code went away for a while and came back. So I replaced the o2 sensor with an oem o2 sensor

It was fine for about 500km and now the code is back. I'm stuck as to what to do next.

There's about 130km on the car (~80k miles)

I spoke to someone at Car Sound and they mentioned the following to check

- temp of the o2 sensors
- spark plugs
- spark plug wires
- engine coolant temperature (ECT) sensor
- head gasket leak (at the engine block itself)
- exhaust leak
- air/fuel mixture

I have DC sports stainless steel headers and a brullen cat-back exhaust with 2.25" stainless steel piping

Any suggestions?