For the record, here's a description of what the computer checks for:
<http://www.smogtechinstitute.com/smogtech/images/PDFs/OBDIIDataInterpretation.pdf>

1. The computer looks to see if the heated oxygen sensor voltage rises
above 600mV and falls below 300mV and switches in less than 100ms.

2. When the cat is working properly there is almost no switching action
from the rear oxygen sensor.

3. To test the rear oxygen sensor, the computer forces a rich or lean
mixture that the cat can't compensate for, and then monitors the
oxygen sensor voltage.
a. Rich to lean threshold voltage
b. Lean to rich threshold voltage
c. Low sensor voltage for switch time calculation
d. High sensor voltage for switch time calculation
e. Rich to lean sensor switch time
f. Lean to rich sensor switch time
g. Minimum sensor voltage for test cycle
h. Maximum sensor voltage for test cycle
i. Time between sensor transitions