If your vehicle is showing fuel gauge always reads empty even after filling up, here are the most likely causes and the parts you'll need to fix each one.
The variable resistor tracks on the sending unit wear out over time from the float constantly moving. This creates an open circuit, causing the system to default the gauge to the empty position.
If the fuel pump was recently replaced, the float arm may have been bent or pinned against the side of the fuel tank or an internal baffle during installation, physically preventing it from rising with the fuel level.
The electrical connector at the top of the fuel tank may be heavily corroded, damaged by rodents, or simply left unplugged after a recent tank drop, cutting off the signal to the instrument cluster.
The tiny electric motor inside the instrument cluster that physically moves the fuel gauge needle has failed, leaving the needle permanently stuck at its bottom resting position.