is the rear o2 sensor needed??? after the cat?

i guess i need more details

i tried to calibrate my wideband (i didnt install it) but i oulled out the rear o2 sensor instead and the threads on the sensor were fucked...so i just put a bung in its place..but my car runs fine without it and pulls the same way...so is it needed?

 

its an OBD II car, it measures catalyst efficiency
when ur cat goes bad it will turn a lite on, but since u dont have it hooked into the exhaust, it will never throw a lite nor will u ever kno ur cat is bad. until the catalyst material goes flying out ur tailpipe.