Toyota Repair: P0420 in Corolla 01, o2 sensors, driveability problem

Question
Hi Ted,

Our Corolla CE 01 has 115K miles (automatic), and since we moved here (West FL) had a pinging problem (never happened in Miami). Regardless the gas station we use, there is a tendency to ping when cruising on the highway at 70 mph (although we notice a little difference from one gas station to another). Nobody have been able to figure out what it might be. Lately, the MIL came on, took it to have the code read, and got P0420, which would indicate a bad converter.
So seems we need to take it to the shop to have the converter replaced.
I believe the code is just the last simptom of a different problem that might be caused by what's causing the pinging. The dealer is asking about $1000 to replace the converter and O2 sensors, but don't want to give them a GO without knowing what might have damaged the converter.
My concern is replacing the converter will not fix it but have another converter burned in few months ahead.

Is there anything I can do to make sure the problem is what they say. I mean, they only read the codes, and bum! we have to change what the computer says. I believe it's a deeper problem, and this is just another band-aid.

Beyond the pingning there is no other driveability problem with it. It has been regularly serviced, and had induction and injector cleaning at recommended times. Gas use is still as new (about 300 miles/tank)
Any hint would be greatly appreciated.
Rich

Answer
I just checked with Toyota's technical information system and there are no service bulletins issued for this trouble code, it seems somewhat unusual but not improbable for the catalytic converter to fail since DTC PO420 indicates there is a catalytic converter malfunction, at this mileage it is very possible that this is the case.