If the downstream O2 sensor is still switching, it won't produce an O2 code because the PCM doesn't see a fault in the O2 sensor. Instead, it determines the fault to be with the catalytic converter.
As BC_ZX3 said, if you have the ability to do so, monitor the switching of the upstream and downstream O2 sensors.
If they appear to be switching at a relatively similar rate, it's most likely you have either a faulty catalytic converter or faulty downstream O2 sensor. More often than not, replacing the downstream O2 sensor will fix the issue.
There is a madness to my methods!!