What could cause the gas gauge on an 2003 Chevy Trailblazer to malfunction needle is not between empty or full after filling up?

The sending units on these vehicle has been a continual problem. Mostly if your vehicle has a history of being in a corrosive environment. Corrosion eats away at the unit as it sits on the top of the tank. It is attached to the the fuel pump and is a big job to replace unless you have access to a lift.