(Updated on July 29, 2021)
There are two kinds of people in the world: those who fill their vehicles up when they start to get a little low, and those who regularly see that “empty” warning light on their instrument cluster. Which are you?
It’s not good to let the fuel tank run below about a quarter tank too often, since then the fuel pump can overheat and the pump, filters, or injectors can clog because the pump picks up more sediment at the bottom of the tank. Getting stranded after running out of gas isn’t good for you or the car.
Drivers rely on an accurate fuel gauge to get where they need to go and take care of their cars. What do you do if the gas gauge is fluctuating, stuck on full, or always reads empty?
How Does a Fuel Gauge Work?
It’s useful to understand the exact way vehicles tell drivers how much fuel is in the tank. The system consists of three parts: the gauge, sender (also called sending unit), and circuit.
The fuel gauge is the part on the dashboard that indicates how full the gas tank is from full to empty. It’s part of the instrument cluster and is connected to the sending unit either by a physical wire or by a wireless connection.
The sending unit is the physical assembly located in the fuel tank that senses the level of the fuel and sends the information via electrical signals to the gas gauge or car’s computer. It’s a part of the fuel pump module, which also includes the fuel pump, fuel strainer, and fuel filter.
The sender is made up of a buoyant float connected to a variable resistor via a wiper (a thin metal rod). The resistor is connected to an electrical ground at one end (either the top or bottom, depending on the vehicle).
The wiper slides up and down the resistor as the fluid level changes, along with the float level. When the wiper is closer to the electrical ground, the resistance is lower since there’s less resistant material for the current to travel through, and vice versa.
This voltage usually comes from the car battery (which releases a steady voltage) and the resulting voltage (that has been affected by the resistor) goes to the computer or gas gauge itself.
The circuit is the wiring that connects the car battery, sending unit, gauge, and ground (which is either the body or frame of the vehicle or the electrical system).
Causes of a Fuel Gauge Not Working Correctly
Since there are three parts that are needed to show the driver how much fuel remains, an inaccurate fuel gauge is always caused by one of them.
1) Failure of Sender
This is the most common problem since there is a lot of motion inside the unit. Parts can get disconnected, worn, or corroded.
The contacts between the wiper and resistor are always rubbing together. If they wear, a correct signal isn’t sent and the level reads as an unchanging full or empty.
If the float physically separates from the wiper, the level always reads as empty.
A faulty resistor can cause erratic readings on the gauge.
An old and sticky wiper gets stuck at certain levels on the resistor and can then get bumped back into place over time or on some rough road. This results in strange fluctuations of the fuel gauge.
2) Issues with Circuit
A problem with the wiring means that the signal can’t get from the battery to the gas gauge because of an issue along the way.
There can be a problem with the wires receiving a signal from the battery, with the ground from either the battery or the sender, or with the wires between the sender and the gauge itself.
The electrical system of vehicles is well-protected, but wires can still corrode or even break if something snags them (like if the vehicle runs over debris).
3) Failure of Gas Gauge or Instrument Cluster
Depending on the vehicle, the gas gauge may be a lone part or it could be a piece of the instrument cluster. Regardless, the internal circuits can be problematic so that the signal is distorted or doesn’t come through at all.
Since electricity is involved, the gas gauge and/or instrument cluster have fuses that can blow. These are easy and cheap to replace, whereas the entire instrument cluster replacement is rather expensive.
How Do I Know if My Fuel Gauge is Accurate?
A functioning fuel gauge should go down at a steady level, depending on the distance and type of driving, after a fill-up. If the fuel gauge indicates the tank is full for more than 75 miles or reads erratically, there may be a problem.
Similarly, if the gauge drops to empty very soon after filling the tank, it should be checked out.
On cars about 20 years old and newer, there may be a self-test procedure for the instrument cluster that will check that the gauge can move from full to empty. This process is different for every vehicle and can be found in the owner’s manual.
The owner’s manual also states how many gallons/liters of fuel the tank holds in your vehicle, and might state the fuel efficiency. You can calculate approximately how far you should be able to go on a full tank by using those two numbers.
For better accuracy, it’s best to have an idea of what your vehicle’s fuel economy is by regularly tracking the number of miles between fill-ups and dividing that by the number of gallons of gasoline that was pumped in at the next fill-up. Tracking this can help you notice a number of problems in the car before they become a big issue.
Can You Calibrate a Fuel Gauge?
Modern car manufacturers can use a computer in the car to change the movement of the fuel gauge a bit by comparing the position of the float to a calibration curve that compensates for the shape of the tank. Fuel tanks are often an odd shape for a more compact design, squeezing between the other parts of the vehicle.
This calibration is also why driving up and down inclines doesn’t rapidly change the needle on the fuel gauge as quickly as gravity moves the float itself – the car has been programmed to know that the fuel level isn’t actually changing that quickly.
It is possible to calibrate a fuel gauge yourself, though it’s not a good idea unless you have an exact reason and specific instructions for your car. You may be able to learn the procedure from an expert and calibrate the gauge by using a multimeter, resistor, and a power source. If you are unsure, leave this to a professional so that the car isn’t damaged.
How to Fix a Gas Gauge
Depending on the problem, a faulty fuel gauge can be either quick and easy or nearly impossible to fix yourself.
First, check all of the electrical fuses. There is usually a box of fuses under the steering column inside the car and another under the hood. If any appear damaged, replace them.
If the self-test procedure for the gas gauge shows a problem or any other instrument cluster lights don’t work properly, the cluster itself may need to be replaced.
Examine all of the wiring between the battery and the gas gauge and the fuel tank. Replace or repair any broken or corroded wiring.
If none of these solve the problem, the sending unit is probably the problem (as is most common). First, try using a bottle of fuel system cleaner in the fuel tank in case some part of the sender is soiled or corroded. Follow the instructions on the bottle.
If that doesn’t work, the sending unit probably needs to be replaced. This is tricky to do at home and is best left to professionals, since it’s difficult to access inside the fuel tank. The tank itself needs to be drained and removed before working on the sender.
Replacement of the sending unit can range between $200 and $800, depending on the vehicle. In some vehicles the fuel tank is more difficult to access so labor costs will be higher. The mechanic may recommend replacing the fuel pump as well since it is usually in the way.
It’s not a great idea to drive around for long with a fuel gauge that isn’t working correctly, since you may be stranded or the fuel pump may pick up too much sediment and overheat.
This may be a problem you can address for relatively little cost. Understanding the system is very helpful, and then some troubleshooting at home can point you in the right direction for repair.