is it a fuse when the gas gauge doesn't work and where is fuse located

I don't have an answer but i have the same problem with my 2004 Fx35. I was told by my mechanic that its a gauge in the gas tank itself. It was expensive( several $100's) to fix so I just keep track of my mileage when I gas up.. It's a pain but I can't afford to get it fixed now. I'd be interested to hear from someone who knows specifically what the issue is....