2001 Mazda 626 catalytic converter

I have a 2001 Mazda 626 EX V6 with 85000 miles. Recently the check engine light has been on. I have had two mechanics check the problem. Both of them say that the check engine light code is PO421 which tells them that it is a faulty catalytic converter. Could there be other problems or should I just replace the catalytic converter.