P420 code after replacing O2 sensor

I have a 2004 Toyota Sienna with 139,000 miles. About 1 year ago the CEL came on with a P420. I reset the code and it remained off for about 6000 miles. Reset the code again and stayed off for about 3000 miles. Reset the code again and then came back on after about 1000 miles. About 1 month ago after reset the light would come back on after about 50 to 60 miles of use. I decided to take the easy path and replace the O2 Sensor after the Cat on Bank 1, which I understood to be the Rear Cat against the firewall). Well after about 60 miles the CEL(P420) came back on again.

So does this mean that I have a bad CAT? Is there something else I can check to make sure. I have a basic OBDII code reader. Do more advanced code readers tell me more?