My grandpas Duramax has 135,000 and the fuel gauge just stopped working. It will read fuel as soon as you turn the truck on then the needle starts bouncing around and goes to empty and the light comes on. We replaced the gauge cluster and it still does it. Does it sound like the sending unit it going bad, Or already gone bad? He has a good cluster if anyone is interested lol.

 

Sounds like a bad sending unit to me.