The fuel gauge on your car's dashboard is meant to provide you with accurate information about how much gasoline is in your tank. Without this handy tool, you might find yourself stranded in the middle of nowhere without a single drop of gas in your tank, and the same can be said if the gauge malfunctions.
Whether a fuel gauge indicates your gas tank is emptier or more full than it actually is, or the needle does not move from "E" anymore, this could present a major safety hazard and be a costly problem to ignore. If you are unsure whether your gas gauge is malfunctioning, there are a few ways to test the part for problems. It is helpful to understand how the gauge reads the tank's levels.
In older systems, the sending unit - a float mounted on a hinged arm - would mechanically move the gauge needle as the level of fuel in the tank fluctuated. Most vehicles manufactured since the 1980s have digital gauges. The same basic principal is used, but instead of a direct connection between the floater arm and the gauge, the sending unit is connected to a variable resistor. As the float travels up and down, it moves a wiper along a thin strip of conductive material on the resistor. When the fuel level drops, the wiper moves closer to the grounded end, creating more resistance. When the resistance increases, the current traveling to the fuel gauge decreases and the needle drops.
One problem that occurs with nearly every fuel level indicating system is that the float will not begin to move the wiper along the resistor until the fuel level dips below a certain point. You may notice that your gauge reads a full tank for a decent stretch of driving after filling up. A similar issue occurs when the fuel levels get too low. The float arm may not extend far enough into the tank to read the levels accurately, so your gauge may indicate the tank is empty when there is still a gallon or two in the tank.
The inaccuracy of the reading may also be attributed to the shape of the tank, as many fuel tanks are constructed to fit Tetris-style into the car. A reading on an asymmetrical tank shape could be accurate as far as the system goes, but the shape of the tank may mean there's actually more or less fuel than the readout.
If you notice your fuel gauge needle does not move, or if it jumps around erratically, there may be other issues at play. The first and easiest thing to check is the fuel gauge fuse. The owner's manual will indicate which fuse is responsible for this system. This is typically the cause of a fuel gauge needle that does not move from the empty position when you start the vehicle.
You may need to test the sending unit of the gauge system. In many models, the unit is easily accessible under a rear seat, but check your owner's manual for the exact location. If the unit is within reach, be sure to disconnect the car battery, starting with the ground wire, before you begin. You will also need to take other safety precautions, such as extinguishing any fires or cigarettes and working in an open-air environment, as the fumes from the gasoline are flammable and can be dangerous.
Once you've exposed the access cover for the fuel tank, unscrew and remove it. You should be able to see the sending unit. Some are bolted in place and others are twist-locked. You can remove it to inspect for damages and test it with an ohmmeter. The owner's manual will indicate the appropriate readings for full and empty tanks.