[tractorboy]

the 02 sensor , or ox sensor. It is on the exhaust down pipe , in back of the engine on most cars. The cat is the catalytic converter. They can become clogged and will choke off the engine. I just recently learned from another poster that if you unscreww the o2 sensor , it gives the exhaust gasses somewhere to go and the car should not atleast die out while idling. Yes the computer will throw a code. The car should not be driven like that, just for testing. There is a pressure guage that is real cheap you can screw in in place of the o2 sensor that will measure the back pressure for a definative answer. oh btw , make sure the ignition is off when umplugging or plugging back up and the 02 sensor is delicate, do not touch the end and silicone as in silicone spray will destroy it, probably a lot of other cleaners too.
you may also need a special tool to unscrew the 02 sensor, one such tool looks like a spark plug socket with a cut out down one side. of course it could be something entirely different, just one idea.