I have a 2000 grand prix gt and I just changed the upstream oxygen sensor.  I noticed the harness that comes from the engine has another connector attached.  This connector isn't attached to anything. Any ideas what it connects to?

not to stress Nick, it will be a generic wire loom and as long as everything you see under there is plugged in and no engine lights are coming up on your dash, your good to go👍

its a good idea to replace the o2 sensors as, if they fail, the computer goes into "default mode" and while the car will run, its will suck gas !.

if your gp has a down stream o2 sensor, change it too mate..

 buy/ borrow your self a obd2 diagnostic tool and you can check that your sensors are working correctly. they are cheap to buy now and can give you heaps of info on what the sensors are reading.( plugs i under the dash, steering column on most cars)