What's Wrong With Your Fuel Gauge?

One of the most commonly-referenced gauges in a car is the fuel gauge. Even people who have never touched a wrench before in their lives and know absolutely nothing about the internal mechanics of cars are aware of what the fuel gauge does. It's important to know when you are starting to run low on gas, after all, and the fuel gauge is how you know when it's time to fill up.

However, just like anything else on a car, it's possible for your fuel gauge to break. Obviously, if you have just driven 200 miles and your fuel gauge still reads completely full, there's an issue. (This would be a pleasant issue to have were it true, but it's not.) Alternatively, if you have just driven off the gas station parking lot from a fill up and your fuel gauge reads empty, that's also a rather obvious problem. Even though figuring out that there's something wrong with the fuel gauge isn't terribly difficult, diagnosing the reason behind your fuel gauge reading incorrectly can be somewhat time-consuming. You may automatically assume that the issue is with the fuel gauge itself, but this is not necessarily the case.

How Does a Gas Gauge Work?

The best way to explain the mechanics behind a gas gauge is to visualize a toilet. Inside of your toilet’s tank there is a float arm. This float arm is how your toilet knows to shut off the water valve after you have flushed and the tank has refilled. When you flush a toilet, the water rushes out of the tank into the bowl, and the float arm lowers with the dispensing water. The toilet’s water valve is open at this point and water will flow back into the tank of the toilet. When the float arm reaches a certain height, the valve shuts off and water stops flowing.

Your gas gauge works in a similar (more complex, but similar) way. There is also a float-like instrument inside your gas tank which communicates with the electrical system in your car. When your fuel level is full, the electrical resistance in the car changes, and this is what causes the gauge to read as full after a fill-up. Depending on the level of electrical resistance, the fuel gauge changes to reflect the level of gas in the tank. In the event that the fuel gauge constantly reads empty, full, or some random nonsensical reading, then there is an issue with the fuel instrument.

So, basically, the issue may be directly with the fuel gauge, but it could also be with a number of other systems that interact with the fuel gauge. It's important to test everything to accurately pinpoint the problem.

How to Test a Fuel Gauge

1

Determine Instrument Cluster Self-Test Procedure

The easiest way to begin is to look in your owner's manual and figure out how the instrument cluster self-test procedure is done. Depending on the age of your car and the model, the process for this varies. Once you have figured out how to activate the cluster self-test procedure, go ahead and perform it.

In newer cars this typically involves turning on and off the headlights a certain number of times and pressing the odometer button; in older ones turning on and off the ignition a few times is generally required.

The fuel gauge needle should go through a full sweep during the cluster self-test process. If it does not, the issue is with the fuel gauge directly. However, if the fuel gauge needle does its sweep as it is supposed to, you need to perform further tests to properly diagnose the problem.

2

Check the Fuses

The next step after performing the cluster test is to check the fuses. Particularly if you are experiencing an issue with more than one gauge on your vehicle, the culprit may be a fuse. If the fuse for the instrument panel has been blown, make sure to replace it. Then go ahead and turn on the car. If the fuel gauge doesn't sort itself out automatically, run the cluster test again.

3

Check the Gauge and Sending Unit

If the issue is not a fuse, then it's time to check either the fuel gauge itself or the fuel sending unit. You should choose the unit that is easiest to access on your vehicle first, and the easiest way to figure this out is to consult the repair manual and see which option looks less headache-inducing. You may end up needing to check both of these, but if the problem is with the easier to access component, you'll be happy that you didn't waste half a day trying to deal with the more-complicated option.

4

Test Electrical Resistance in the Gas Tank

If you test the fuel sending unit first, first you need to make sure that the tank is completely full. Then you need to use a multimeter to test the electrical resistance in the tank. Make sure to consult your repair manual so that you understand what electrical conductance is used to work the gauge, as this varies between make and model of vehicle. If the resistance appears to be off, try using some fuel system cleaner. It is possible that the float on the sending unit has been corroded and cleaning it usually solves the problem.

5

Test the Gauge with a Multimeter

To check the fuel gauge itself, remove the instrument cluster from the dash and test it with the multimeter. Assuming that the wiring that is attached to the gauge is receiving 12 volts, the issue is likely with the gauge itself and at this point it needs to be replaced. If the wiring does not read 12 volts, then you may have a wiring problem.

6

Check the Wiring

Testing the wiring is not particularly difficult, but it can be time-consuming. You need to visually inspect the wiring and deduce potential problems with it. For instance, it's possible that the wiring has become loose, dirty, corroded, or have worn down insulation. Depending on what the particular issue is, you may need to clean, tighten, or even replace the wires.

As always, if you run into a problem that you can’t fix, go ahead and consult a trusted mechanic. However, with a little bit of patience, fuel gauge issues can easily be sussed out and repaired, even by amateur do-it-yourselfers.

Shop All Gauges

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