1. The Most Likely Culprit: Fuel Level Sending Unit
* Location: This unit is located in the fuel tank. It's a float mechanism that moves with the fuel level, sending a resistance signal to the gauge. A faulty float, a broken wire connecting to the float, or corrosion in the unit are common causes of an inaccurate reading.
* Troubleshooting:
* Check the Fuel: The most obvious first step! Make sure you're actually low on fuel. Run the tank down significantly to see if the gauge moves. If it *still* reads full, the problem isn't just a simple misreading due to a full tank.
* Visual Inspection (Difficult): Dropping the fuel tank is usually necessary for a proper inspection. Unless you're very comfortable working on fuel systems (it involves potential fire hazards), this is best left to a mechanic. Even if you can see the unit, you likely can't test it properly without tools and a wiring diagram.
* Repair/Replacement: The sending unit is usually replaced as a whole unit. This requires dropping the fuel tank, which is a significant job involving disconnecting fuel lines (carefully!), electrical connections, and tank straps. A new sending unit is relatively inexpensive, but the labor is what makes this repair costly.
2. Other Possible Causes (Less Likely):
* Fuel Gauge Cluster: The gauge itself could be faulty. This is less likely since it's consistently reading full, but it's possible there's an internal short or failure in the gauge. Testing this would require an ohmmeter to check the resistance values at the gauge connector.
* Wiring: A break or short in the wiring between the sending unit and the gauge cluster could cause an inaccurate reading. Tracing the wiring harness for damage is difficult without a wiring diagram.
* Ground Issues: A poor ground connection anywhere in the circuit could affect the gauge's reading. This would require checking ground connections throughout the fuel system circuit.
Recommendation:
Unless you're experienced with automotive repair and comfortable working with fuel systems (which is very important due to fire hazards), I strongly recommend taking your truck to a qualified mechanic. They have the tools, experience, and safety equipment to diagnose and repair the problem safely and effectively. Attempting this repair yourself could lead to damage, injury, or fire.
Before taking it to a mechanic: Try to note *exactly* when you last filled the tank, and how much you estimate you've used since. This information will help the mechanic narrow down the possibilities.
