First thing to check is the performance of the cat.
A PO420/430 does not automatically mean a defective cat,all the code means is the cat is performing below its optimal efficiency for some reason.
The first thing to do is tap the converter body and make sure there is nothing rattling inside and there is no rust holes then get the engine and cat up to operating temp.
Next,bring the engine up to approx 2500RMP and use either a digital thermometer (high temp)
or a non contact infra-red thermometer on the inlet,body and outlet of the cat.
The temp should be higher at the outlet than the inlet.If you have around 100f @2500 higher at the outlet the cat media is functioning and the cat is probably fine,at idle late model cats can have as little as 30-50f difference at idle.
The cat body temp should be higher than the inlet getting hotter as you get closer to the outlet.No 100% guarantee the cat is good but at least good enough to warrant further inspection.
Make sure there is no leaks at all between the front O2 and the rear O2.The smallest of leaks in this area will throw a cat code.
On cars with a doughnut and spring set up on the exhaust manifold this joint needs special attention.
I have seen a few cases of this joint leaking slightly under certain conditions e.g. under hard deceleration.
What happens is the engine rolls slightly forward and the joint acts like a ball joint(its designed this way) so it doesn't break the pipe.
Because its under deceleration and the movement is so small the operator does not hear an exhaust leak but there was additional outside air introduced into system after the front O2 causing the PO420/430.
Weakened springs from corrosion or just age is the most common reason for this happening.The repair for this is easy,a new doughnut,bolts and springs will cure it.
A scan tool that has the capability to read the O2 sensors in real time is needed to diagnose a lazy sensor.
A failed O2 will set a code but many times a lazy sensor can set an PO420/430 code.
On engines with 100K and more miles i always change the O2 sensors anyway whenever these codes show up.
The reason is they do get lazy with age and if the cat is bad you would want to put new O2's in the system anyway to get the most efficiency from the system.
If the cat is bad use either OEM or high quality after market e.g. Eastern cat and not universals.
In Asian cars i try to use Denso O2's as they are most likely used OEM anyway,ditto no universals.
Edit: If a clogged cat is suspected there are other testing methods that can diagnose that but seeing as you didn't mention performance issues i didn't get into them.
Any other issues like vacuum leaks,bad MAF or MAP sensors,coolant temp sensors,misfires etc should be addressed before addressing the PO420/430 code.
