Do I need to use an OEM catalytic converter on this car? on 2001 Pontiac Grand Prix

When I bought this used car (100,000 miles)it had a new catalytic converter on it. Shortly after that I got a SES light and the dealer said the catalytic converter had failed again. He replaced it. 5000 miles later I have a SES light and had a local garage I trust scan it. They got code P0420, converter failed. They say the dealer used an aftermarket converter and you need to use an OEM unit, $586 is the friendly price for a Magnaflow unit!

Asked by for the 2001 Pontiac Grand Prix
An OE cat must be used in California legally unless the "new" cat is CARB approved. OE cat's have more catalyst material in then to neutralize harmful exhaust gasses but the bigger problem is what has anyone done to address what is causing the cat to fail in the first place? Is the engine running too rich or way lean, is the head gasket failing or cat getting oil contaminated? Could it be the O2 sensors which monitor cat converter efficiency are bad?
The car runs great, looses no coolant, does not use oil, and does not appear to run rich. I will try getting the 02 sensors checked.