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  • A vehicle’s fuel tank has a sending unit with a “float” that floats on top of the fuel supply at all times, remaining on the surface of the gas in the gas tank and indicating fuel levels to the fuel gauge. A faulty sending unit will result in an inaccurate fuel gauge.
  • Symptoms of a faulty fuel sending unit include the fuel gauge getting stuck on empty or full and the fuel gauge behaving erratically.
  • You can check your fuel levels even with a faulty fuel sending unit by learning the capacity of your vehicle’s fuel tank and keeping track of the miles you’ve driven since your last fuel top-up. However, it’s still best to get your fuel sending unit repaired as soon as possible.

If a vehicle has a fuel tank, it has a sending unit with a “float” that is either a brass capsule filled with air or a floating foam part that floats on top of the fuel supply at all times. That’s why it’s called a “float.”

The float will, by design, remain on the surface of the gas in the gas tank because a float always floats unless it leaks and/or absorbs fuel.

The float is attached to a metal rod that operates a variable resistor connected by electrical wiring to the fuel gauge. The amount of current that’s able to pass through the variable resistor to ground varies depending on the float’s height.

diagram of a fuel gauge sending unit
Diagram showing how the fuel gauge sending unit is usually part of the fuel pump assembly. | Image Source: Richard McCuistian

Symptoms of Faulty Fuel Sending Unit

You might notice that your fuel gauge is not correctly reading the amount of fuel in your fuel tank. If this is the case, then your vehicle’s fuel sending unit might be at fault. It can be the gauge, but the fuel tank sending unit is in the most hostile environment, is doing the most physical work, and is the most likely to fail.

Most gauges have some type of electronic “anti-slosh” function to keep the fuel gauge from moving around as the fuel sloshes. Typically, the fuel gauge won’t correct quickly after filling the tank unless the key is cycled.

Here are the symptoms of a faulty fuel sending unit:

Fuel Gauge is Stuck on Empty

The float may break off or separate from the arm. There might also be an issue with the variable resistor. An empty reading should only occur if the resistor is either creating the maximum amount of resistance or not allowing current to flow. However, there are times when the fuel sending unit will completely stop sending current at certain positions.

Fuel Gauge is Stuck on Full

A fuel gauge that is stuck on full is not usually a sign of a problem with the fuel sending unit. This issue is usually caused by a malfunctioning variable resistor that gives a bad signal to the fuel gauge, causing it to read full all of the time. However, there are times when it may indicate a problem with the fuel sending unit itself.

Erratic Fuel Gauge

The fuel gauge should not fluctuate or behave abnormally. When everything is working properly, the fuel gauge will continually move toward empty until you refill fuel.

If your fuel gauge is erratic, it might indicate a problem with the fuel sending unit. If you want to verify if your fuel sending unit is faulty, then you’re going to need a multimeter and access to the fuel tank.

old and new gm fuel sending units
A worn-out GM sending unit (left) and a new sending unit (right). | Image Source: Richard McCuistian

If you don’t have a meter or don’t know how to use one, you can simply disconnect and reconnect the sending unit and watch the gauge, because that’s the first step of what most mechanics do, anyway.

Most sending units have a low resistance when full and a high resistance when empty, so if you disconnect the sending unit and cycle the key, the gauge should go past full. There will usually be three wires in the fuel pump gauge connector, and you need to know which wire feeds the fuel gauge. 

Two of the other three wires will be grounds (one for the gauge sender and the other for the fuel pump) and the third wire will give power to the fuel pump. But the fuel pump ground and power feed are usually a heavier gauge wire than the fuel gauge wires (although on some vehicles they’re the same size).

Be careful not to confuse the fuel gauge/pump wiring with the fuel tank pressure sensor, which usually only has 3 wires and is sometimes mounted on the same plate as the fuel sending unit.

On many vehicles, the fuel tank sending unit must be accessed from beneath the vehicle rather than simply raising the seat or removing carpet from the upper side of the trunk.

This can be a laborious task depending on where your fuel tank is located. If your car’s fuel tank is underneath your rear passenger seat, you may have to remove the floor trim and carpeting before you see the top of the fuel tank.

How To Check Fuel Levels When You Have a Faulty Fuel Sending Unit

You won’t be able to know how much fuel remains in your fuel tank if the fuel sending unit is not working. Luckily, you can use the following methods to estimate the amount of fuel remaining in your tank.

The first thing you should do is find out the capacity of your vehicle’s fuel tank. You can check your owner’s manual or use reputable online resources to get this information. Knowing how much fuel can fit in the tank can help you estimate your vehicle’s remaining range.

Another way you can estimate your vehicle’s remaining fuel is by keeping track of the miles you’ve driven since you last filled up the tank. Any vehicle has an estimated max range depending on its fuel capacity and fuel economy. While you might not know your vehicle’s exact maximum range, you probably have a sense of your vehicle’s fuel economy.

For example, assume that your vehicle has a fuel economy of 25 miles per gallon Knowing that your car has a 12 gallon fuel capacity, you can reasonably estimate that your vehicle has around 300 miles of range left. You can then estimate how much fuel you have remaining in your tank depending on how many miles you’ve driven. Of course, it is still important to get your fuel sending unit fixed as soon as possible to remove the guesswork from figuring out how much fuel you have left.

How Much Does a Replacement Fuel Sending Unit Cost?

A brand new fuel sending unit can cost anywhere between $25 to $800. The price is affected by factors such as the fuel sending unit’s brand, design, set inclusions, and intended vehicle.

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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