Toyota Repair: P0420 error code, toyota 4runner, o2 sensors

Question
I have a 2000 toyota 4runner and it has a P0420 error code, indicating a bad catalytic converter.  I have replaced the cat and still have the code, even after reseting it.  What else could I look at to clear up this code?  Also my vehicle came from California and I have heard there are more stringent emissions there.  Is it possible the computer is set to read leanner emissions?

Answer
Was the replacement converter a factory OEM unit from the toyota dealer? If so, the replacement is exactly as the original for the state the vehicle was sold in because the replacement is matched to the VIN#. If it's an aftermarket converter then that could be a problem, the best way to verify whether there is a cat converter malfunction is to check the exhaust gas readings with an emission analizer so I would recommend doing that, there is also another test that can be done using the readings from the O2 sensors from in front and behind the cat to see if it's working.