gas gauge not working. the needle go up and down, then stays on full.

The sending unit for the gauge is part of the fuel pump module inside of the gas tank. This pump module needs to be replaced. This involves tank removal. Will need to be done at a repair shop. Save a little money if you run the fuel level down to about 1/4 tank if possible. This way the tank is easier to remove. Also should be no charge for drain and refill that way.