what year is the car? just want to know if its obd2 or obd1

anyways, u need a scanner to diagnose it.

u need to do a swing test on the o2 sensor and it will isolate the catalytic converter.

but usually, in this case, ur cat is degraded already.
if the post 02 sensor passes the swing test then u will need a new cat but do not change the post 02 sensor