O2 sensor on a 2002 Mazda mpv

Catalytic converter must be clogged.

Can't be I hollowed out the catalytic converter

Air/fuel mixture is off.

There is a whole multitude of reasons for this. From bad/clogged EGR to dirty fuel injectors. Vacuum leaks, Running to rich to running to lean. A bad spark plug to burnt valve. Low fuel pressure from pump to dirty fuel filter, basically anything altering the fuel/air mixture. Most often running to lean & ignition timing which in this case is computer controlled so it has to be that the computer is not getting the correct information to determine the correct mixture or is hindered in being able to correct it. Possibilities could also be electrical connections to sensors that the computer rely on. The computer could be in limp mode, which could be computer issue if all others are in good working condition. I would test what can be tested and clean all that can be cleaned like the PCV, EGR & test it, Throttle body, IAC, pull fuel injectors and test or rebuild even if its just the small filters and o' rings kit, and fuel filter/air filter, fix all vacuum leaks, test fuel pressure, If check engine light is on scan code to see which system is showing up to test or clean and fix those issues related to that code, if all else fails put in a known good computer to test with.

I had the same problem years ago on my 02 MPV and I believe my mechanic changed the plug wires. He said that a cylinder wasn’t firing. The problem was solved.