how do you know if you have a bad o2 sensor...what are the symptoms...and would that have a effect on your car...

i just installed a high cat...and was just wondering if that's the cause of my checkengine light...anyone know...oh yeah and would that mess up my sfmu

An O2 sensor will act sluggish as the ECU cycles the motor back and forth between rich and lean. It will give you poor part throttle fuel economy, and will lightly effect performance (only while in closed loop). Or it will be stuck at one side when it's completely bad, causing permanent open loop.

A key dance or scan tool will tell you the cause of the CEL.

if i install a high flow cat would that effect my o2 sensor readings

It will effect the rear 02 sensor reading, which is used on our car to only check against the front O2 sensor to see if the cat is working.

Some say it retards timing when it's having a problem (which would heat the cat, but raise emissions), but I have not seen evidence of this myself from before and after removing the rear O2 sensor when I had a stock ECU.