[A4] Catalytic Converter

[Brizax at aol.com]

there are 2 o2 sensors, one before and after the Cat. if the cat fails and  
they both read the same o2's it trips an CE light.  Otherwise the one after  
the cat reads lower voltages "leaner" than the pre cat o2 sensor

Someone  correct me if I'm wrong, as I 
My understanding of the OBD-II diagnostics  is that it reports
about the sensors in the system, whether they are  working and
within range or not.  What sensor is there that would  finger
a catalytic converter as being bad? That is, what does the
OBD-II  diagnostics test to determine the cat is bad.