The aft O2 sensor measures the ability of the cat to store oxygen. Just because you got this code, it doesn't necessarily mean that the cat has failed. Resetting the code doesn't make the problem go away. For a first step, I would swap the two aft O2 sensors to see if the problem moves to the other bank -- while doing this, inspect the wiring for any problems (if it was the O2 sensor, then you'd get a P0420 DTC). Also, check the exhaust around the cat to see if there's any small holes as these would let in air and skew the O2 readings.
Please provide year/mileage on car.
Regards,
paul...
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