How do you tell if gas gauge is bad?

If it reads full and you run out of gas, haha Seriously, though, there are really two main reasons why your gauge would be bad. The first, and most common is a bad float/sending unit. The float/sending unit sits in your gas tank, and sends a signal down a wire to your gauge The signal varies in voltage based on how full or empty your tank is. Most of the time, this can only be replaced with a whole module (comes with float, sender [the part that transmits the signal], wiring harness, and fuel pump) at between $150-300, depending on the vehicle. This is all assuming that your vehicle is pretty current. If it is an antique, or collectible it may have a separate sensor that sits in the tank. This is much easier and cheaper to replace, but it may be harder to locate the part for purchase.