Your gas gauge is one of those things that you usually trust implicitly. Without it, you’d probably end up stranded at the side of the road with an empty tank. When your gas gauge has a problem, stops working gets stuck in one spot, it can be more than just an inconvenience.
Not only do you run the risk of running out of gas and being stranded, but a very low gas tank in an older vehicle can draw tank deposits and varnish into the fuel system. This can clog the car’s fuel filter, strain the fuel filter, and cause other serious faults leading all the way to the engine’s fuel rail.
To find out why your gas gauge isn’t working, we’ll need to take a closer look at how it reads the fuel level and how it sends that information to the dashboard console.
How a Gas Gauge Or Fuel Gauge Works
Inside your car’s gas tank, there is a special float arm that sits effortlessly on top of the gasoline. It is linked to a special type of electrical resistance-sweep resistor known as a rheostat. As the fuel level gradually lowers or rises, it affects the rheostat, which sends a signal to the gas gauge on your dashboard instrument cluster.
Most modern-day automakers set a low fuel level threshold in the system. When the fuel level gets critically low, it triggers a warning light to come on in the dash, telling you it’s time to fill up immediately.
4 Common Reasons for an Ineffective Gas Gauge
A bad fuel-sending unit is often the main reason a gas/fuel gauge shows inaccurate fuel amounts. This unit is situated within the fuel tank and conveys the fuel quantity to the dashboard indicator. If it malfunctions, it can provide misleading information to drivers.
Nonetheless, it’s certainly not the only reason why your gas gauge isn’t working properly; issues with Corroded Wiring, malfunctioning dashboard gauge, or even Moisture in the Gas Tank in modern vehicles can cause inconsistent fuel level displays. When diagnosing a gas gauge that’s not showing correct levels, it’s crucial to account for these potential factors.
1. Bad Fuel Sending Unit
A faulty fuel-sending unit is the most likely reason why your gas gauge is giving you bad information or no information at all. This floating arm device inside the gas tank includes a rheostat resistor. As the fuel level changes, the tiny electrical resistance changes, and that information is sent on to the gas gauge on your dash console.
Depending on when the fault occurs, a bad fuel-sending unit might cause the needle to stick stubbornly or give a reading that makes it look like the gas tank has more or less fuel than it actually does.
Analog gas gauges are likelier to stick or give you a false reading. This is the classic scenario where the needle is eternally stuck at half a tank or a quarter off from the accurate reading.
A bad fuel-sending unit often causes the gas gauge to not work at all in a digital instrument cluster. However, this isn’t always the case if the float arm has jammed for some reason.
Where Is the Fuel Gauge Sending Unit?
The fuel gauge sending unit is usually sealed inside the gas tank or built into the fuel pump assembly. This makes directly accessing it difficult as well as dangerous for a DIY mechanic. Though there are some ways to test the sending unit to either confirm or eliminate it as the reason your gas gauge is giving you a wonky reading
How to Diagnose a Bad Fuel-Sending Unit
If your gas gauge not working is caused by a bad fuel-sending unit, and you get a check engine light, it might throw one or more of the following codes.
Some of these codes could be related to electrical faults, shorting wires, and/or blown fuses. So before you throw your arms up in the air, convinced that you have a bad fuel-sending unit, take a few extra minutes to check all the fuses related to the instrument cluster. It’s also a good idea to perform an instrument cluster self-test to test all the readings on your dash, including the gas gauge.
The final way to test a bad fuel-sending unit is to remove it and test it with a multimeter. However, this is not the sort of diagnostic that the average DIY mechanic can do independently.
How to Fix a Bad Fuel-Sending Unit
Replacing a bad fuel-sending unit usually requires getting into the fuel tank, which isn’t the sort of thing that the average DIY mechanic can safely do. The wise move here is to bring the car to a trained mechanic with the tools and training to do it right.
The cost to have a bad fuel-sending unit replaced will range from $175 to $450.
2. Moisture in the Gas Tank
Condensation and low-quality fuel can allow trace amounts of moisture to build up inside your gas tank. This is even more likely to be an issue during cold weather when the vapor condenses inside the tank. If the moisture comes in contact with the rheostat resistor of the fuel-sending arm, it can cause a false reading in the electrical signal.
This usually causes the needle on an analog gas gauge to stick in one place, even though you know you just filled up the tank. A digital gas gauge could either give you the same reading it had before fueling up, or it might tell you that the tank is about to run empty. You might even get a fuel warning light minutes after leaving the pump.
How to Diagnose Moisture in the Gas Tank
Diagnosing moisture in the gas tank requires taking stock of all the symptoms. In cold weather, moisture can cause the gas gauge to stick or not work at all. You might also notice the sputtering or erratic moments of hesitation when you step down on the gas pedal in the first few miles after you fill up.
These odd engine performance issues are usually caused by tiny water blobs getting mixed up with the onrush of fresh fuel. Once the tank is disturbed, they can get sucked into the fuel system, where they can wreak havoc.
After a few miles, the tank’s water and gas settle out and separate. The engine eventually smooths out, which can easily trick you into thinking the problem is over. When really the moisture is just waiting to cause problems in the tank later.
How to Fix Moisture in the Gas Tank
If you suspect that your gas gauge problems are due to moisture in the gas tank, pouring some fuel line antifreeze into your tank is the easiest way to handle it. Then top it off with premium high-octane fuel.
Fuel line antifreeze like ISO HEET doesn’t actually remove the water. It truly combines with moisture to reduce its impact as it passes through the fuel system.
Our Product Choice: Heet Iso 28202 Fuel Line Antifreeze + Injector Cleaner
If the weather is warm, and you suspect there’s a lot of moisture/water in your gas tank, you’ll have to have a mechanic completely drain the tank. This will also remove help remove any excess tank deposits. Once the tank’s been purged, you can refill it with premium gas, and the gauge should work like normal again.
The cost to have a mechanic drain your fuel tank is around $75 to $150. Not counting the cost of refilling the tank.
3. A Bad Gas Gauge
The gas gauge itself can fail. This is rare and usually only with older, high-mileage vehicles with an analog needle. You tend to see it more in commercial vehicles like long-distance cargo vans and pickup trucks used for road construction that see a lot of wear and tear.
How to Tell If You Have a Bad Gas Gauge
Diagnosing if you have a bad gas gauge starts with performing an instrument cluster self-test. The test might vary in some makes and models. So, check your owner’s manual or repair guide. However, most instrument cluster self-tests call for the following steps.
The instruments should turn on, warning lights will turn on, and you might get dings or bongs.
If the gas gauge doesn’t light up and/or doesn’t perform any sweeps of the needle or digital display, then the gas gauge in the instrument cluster needs to be replaced.
If multiple components in the instrument cluster didn’t respond to the self-test, then the problem might be bad fuses. If the entire instrument cluster didn’t respond, it might need to be completely replaced.
The cost to replace just the gas gauge or part of an instrument cluster can vary from $175 to $450.
A total instrument cluster replacement can range from $750 to $1,250.
4. Burned Out Fuses or Corroded Wiring
Burned-out fuses and bad wires can interrupt the signal between the fuel-sending unit and the gas gauge. This can seem like a very odd electrical gremlin that will leave you scratching your head.
I had an older truck once where if I opened the driver’s side door too fast, it would blow a fuse that would take out the radio, the gas gauge, the odometer, and the speedometer. Yet everything else in the instrument cluster still worked normally.
It ended up being a corroded wire in the wiring loom leading to the door speakers. It would contact another wire, ground out, and blow the radio fuse, which happened to share the same fuse with the gas gauge in the instrument cluster.
Start by checking your interior fuses. If one is burned out, and you replace it only to have it burn out again a little while later, it usually means there’s a short-circuited or damaged wire somewhere. You can read through your owner’s manual or the repair guide to dial in where it might be. Look for other devices or gauges that might share the same circuit.
You can usually find the interior fuse box near or under the steering column. Though some newer cars will also have another one in the foot well of the passenger side. Then there’s always a fuse box under the hood. When the gas gauge is not working due to a blown fuse, it will be in an interior fuse panel.
Frequently Asked Questions
Can I Drive with a Bad Gas Gauge?
Driving with a bad gas gauge itself won’t kill the car. Running out of gas because you don’t know how much fuel you have can draw tank deposits and moisture into the fuel system. This can clog the fuel filter, pump, or fuel injectors, which can lead to engine damage. So, driving for a long time with a bad gas gauge is just asking for trouble.
How Do You Tell How Much Gas You Have With A Bad Fuel Gauge?
It’s never wise to drive with a bad gas gauge. If you need to get by while you’re troubleshooting the problem or coming up with the money for the final repair, you’re going to have to do a lot of math. This starts with filling up your tank until the service station pump turns off.
Then check your odometer, and look in your owner’s manual to determine the tank size. Then look up your car’s rated city mileage. Then multiply that by the number of gallons your tank holds.
Let’s say you have a 2022 Ford Fusion with a 16.5-gallon tank.
It’s rated to get 23 MPG in the city.
Quarter Tank = 95 miles driven
Half Tank = 198.5 miles driven
Three-Quarter Tank = 284 miles driven
Then it’s best practice to keep the tank at least a quarter full or better until you can get the bad gas gauge fixed.
Conclusion
Most of the time, an inaccurate, stuck, or dead gas gauge on the dash is caused by a bad fuel-sending unit in the gas tank. Since it’s usually integrated into the tank, you usually need a professional mechanic to fix it.
Though you don’t instantly have to throw your arms up in defeat, if the weather’s been cold, there could be moisture in your gas tank, which is affecting the resistor in the fuel-sending unit, causing the gas gauge to stick.
So, adding some fuel line antifreeze like ISO-HEET is worth a shot. Then fill up the tank and go for a good drive. When you’re done, park the car in a heated garage. If the gas gauge had started working again, it might have solved the problem cheaply!
If your gas gauge is not working, and you’ve been having problems with other gauges on your instrument cluster, it might be a burned-out wire, short-circuited wires, or blown fuses. Test all your fuses and do your best to follow wiring looms to look for signs of contact faults.
While you’re at it, look up the specific instructions for performing an instrument cluster self-test. This will help confirm or eliminate if the gas gauge on your dash has gone bad. If all these tests turn up nothing, then it’s probably time to call your mechanic to have them inspect and replace the fuel gauge sending unit.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.