* Faulty Fuel Sending Unit: This is the most likely cause. The sending unit is a float mechanism that rides on the surface of the fuel and sends a signal to the gauge indicating the fuel level. If the float is damaged, the arm is bent, the potentiometer (the electrical component that translates the float's position into a signal) is faulty, or there's corrosion in the connections, the gauge will give inaccurate readings, including the dramatic swings you describe.
* Loose or Corroded Connections: Wires connected to the fuel sending unit can become loose, corroded, or broken. This interruption in the electrical signal will cause erratic readings on the gauge.
* Debris in the Fuel Tank: Sometimes, debris can interfere with the float mechanism of the sending unit, causing inaccurate readings.
* Wiring Issues (Beyond the Sending Unit): A problem with the wiring harness between the sending unit and the gauge itself can also cause erratic readings. This could include a short circuit or a break in the wire.
* Faulty Fuel Gauge: While less common than a sending unit problem, it's possible the gauge itself is malfunctioning. This is less likely to cause the *specific* full-to-empty jump, but erratic readings are possible.
* Bad Ground: A poor ground connection in the vehicle's electrical system can affect the accuracy of various gauges, including the fuel gauge.
To diagnose the issue, a mechanic will likely start by checking the connections and visually inspecting the fuel sending unit. Often, the best approach is to replace the entire sending unit as it's usually cheaper and easier than trying to repair individual components.
