If your vehicle is showing fuel gauge stuck on full, here are the most likely causes and the parts you'll need to fix each one.
The variable resistor contacts on the sending unit wear out over time. Depending on the vehicle's design, this open circuit often causes the gauge to default to the highest resistance reading, pegging the needle past full.
If the fuel pump was recently replaced, the float arm may have been bent or wedged against an internal tank baffle during installation, physically trapping the float in the highest position.
A broken wire, chewed harness, or a connector left unplugged after recent service creates an open circuit. Many vehicle computers interpret this lack of signal as a completely full tank.
The tiny electric motor that physically moves the fuel gauge needle on the dashboard can fail or bind up, freezing the needle permanently in the full position regardless of the actual fuel level.
In modern vehicles, the BCM processes the raw fuel level signal before sending it to the dashboard. A software glitch or internal failure can cause it to output a constant 'full' signal.