PO420 questions

[Hard_Yaris]

Hello there,

i have thrown this code twice now.
I thought i had rocks rolling around in the wheel because it only happened at low speed, about 10km/h. *removed tire and checked*nothing!*still rock sound*
Does this have anything to do with the catalytic converter or O2 sensor?

Also if one cat is bad and there are two on the yaris could i cut one out and replace it with pipe?
This is the way it is right now.
Manifold --- cat--cat--resonator ----muffler
i want to do this
Manifold---cat---resonator-----muffler
Will this configuration damage the motor?

Thanks!