I'm having the same problem with my Dad's 2002 ES 300: P0420 and P0430. It also has the TRAC and VSC lights on, which means there is a traction control failure.
I pulled the O2 sensors out, they look fine. In any event, a P0420 code and a P0430 code both at the same time DOES NOT INDICATE A CLOGGED CAT! The P0420 means the front post cat O2 sensor has detected a reading fluctuation. The P0430 code is rear cat o2 sensor has detected a reading fluctuation. The chance of the same physical problem occuring at the same time to both front and rear is almost impossible.
So there are other reasons why these codes might occur. They are:
* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings
So my problem, along with the VSC light, is leading me to believe I have either an exhaust leak, a stretched timing belt, or the VSC is leaking. Probably the VSC, but I don't understand the damn thing yet.