I have a 2000 Corolla, 200K miles with DTC P0420. It has a new spark plugs, few year old MAF sensor, and two new O2 sensors. The P0420 comes back every time soon after I clear the code with a scan tool. (Actually the code comes back quicker after the O2 sensors are replaced by new ones.) The scan tool cannot make plots, but every time O2 sensor 1 has low voltage (high O2 level), sensor 2's voltage also goes low within a second. I think this means the catalytic converter cannot remove much O2 from the upstream. I think I should still check if there are leaks in exhaust systems and fuel injectors.
If I ended up needing a new catalytic converter, which brand, model would you recommend? I heard that some aftermarket ones don't work very well.