What are the symptoms of a bad o2 sensor?

Answer"Check Engine" light for one (usually).

When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and uses more fuel than it needs to.

It will run too rich or too lean changing from moment to moment.

A bad O2 sensor will cause the catalytic convertor to fail because the engine ran too rich and dumped too much rich exhaust into the cat - cats are very expensive, I just had a new one installed, sounded like it had rocks in it. And of course that assisted in the murder of the muffler. If the cat has failed, odds are that the O2 sensor has too.