How to test secondary O2 sensor behind catalyst converter?

Getting a P0420 reading (catalyst efficiency below threshold) reading. Want to check the O2 sensor before I replace the FREAKIN expensive converter.

Thanks in advance.

What year/model are we talking about here? Do you have any modifications done to your exhaust system? The secondary sensor is just used to make sure the cat is working properly. Usually it doesnt affect how the car runs so you could do something along the lines of what I did and space it further back from the exhaust flow (I replaced the cat with a test pipe). Or usually the secondary sensors are pretty cheap so you could replace that.

in my time of working at a smog shop the cat efficient below threshold is self explanatory cat IS NO GOOD.