04 Cavy Wheel Hub Sensor

[atlas86]

Hello everyone! I'm brand new to the forum, and i guess i'm really just here to get repair info whenever something breaks.
Anyways, my gf's 04 Cavalier has had a broken wheel hub sensor for almost a year and a half. i replaced the same part in my 02 Alero. The question is this: Can I simply replace the sensor itself or do I have to replace the entire assembly (as was the case with my car)? Also, could something like this be severely affecting her gas mileage? I've replaced her spark plugs, tires, and toppped up her fluids and had a oil change but the car's mileage still is rather poor--around 15mpg city and 20 hway. Any help would be splendid.
Pat