O2 sensor question
 

theoretically, what would happen if the O2 sensor was disconected from the ECU? would it run rich or lean? i kinda need to know it for school.

eddie

From what I understand, its only used during "closed loop" conditions like when you are at a steady cruising speed on the highway, or cruising around da hood in 2nd gear. The ECU will sample the exhaust via the O2 sensor to try to come up with the most economical fuel mixture to give you better MPG

You'll use slightly more fuel. That's about it.

For your school report's sake.....rich.
Because the computer isnt sensing any oxygen coming out of the exhaust, therefore it doesnt know what amount of fuel to give.

LIke Project84 said, only on closed loop conditions, on my old Honda my O2 sensor was disconnected and was fouling spark plugs when I would start cruising on the highway due to extremely rich conditions.

Good luck with the paper

Will a bad o2 sensor also lead to premature cat failure due to rich conditions?

It won't run extremely rich. The ECU just refers to its normal fuel maps instead of using the O2 sensor. I'm currently running without an O2 sensor and have noticed no change in performance, visable black smoke or foulled plugs. My fuel consumption has increased ~5%.