Bryan,

I'm kind of guessing here, and I don't know what kind of car you have, but I think you have to have a car new enough to have an oxygen sensor after the cat or the OBD system would not have any idea what the cat is doing. My 95 940 has only one oxygen sensor. I think 96 went to OBD II, but I don't know if that included the dual sensors.

If you have only one sensor, I think the smell of the exhaust and the color in the tailpipe may be the only check you can do yourself. A shop with a gas analyzer can probably tell if it is working.

Charley