i had mine disconnected for a while and it didnt seem to make a difference.. what does it do and do i need it connected?

 

it senses how much oxygen is in your exhaust. and it will probably make your engine run too rich or too lean because it senses hella oxygen. i'd keep it in if possible

 

You need it. It tells your ECU how you're running (Rich, lean, stoich). You will either be running really rich or really lean w/o it, and are probably throwing codes. Depening on which engine, you will also be in limp mode until you get it taken care of. If you're wondering if it has any type of "performance" gains from taking it out..it doesn't. The car will run w/o it, but you should notice a difference when you put it back in and reset your ECU.

jB