When you get in your car, you assume that each of the gauges and lights in front of you will function normally. That can make it more than a little unsettling when one of those helpful gauges doesn’t work as it should.
So, what do you do when your fuel gauge reads incorrectly, and what does that mean? More importantly, how do you fix it?
How a Fuel Gauge Works
When you turn the key to the ON position on older vehicles (or start them), you see the fuel gauge go to full, back to empty, and then back to a position that marks how much fuel is in the vehicle. Modern vehicles have a digital display instead, but they work much of the same way. It all starts with the fuel tank.
Inside the fuel tank is a component called a float gauge or fuel sending unit. It works a little bit like a fishing bobber—the buoyant portion is on top of the fuel, and it’s attached to a two-piece metal rod that has a split at the centre.
This split allows the rod to bend, so the whole gauge becomes a mechanical arm that can keep the buoyant part on top of the fuel whenever your tank is mostly empty or full.
The float gauge measures the bend in the arm, and it turns this bend into an electrical signal to be sent to the fuel gauge.
When fully extended, the float gauge sends a signal that the gauge interprets as gas level FULL, while one that is almost completely bent in half sends a signal the gauge interprets as gas level EMPTY.
Everything else is somewhere in between, and that is how the gauge knows what amount of fuel is present in the tank. The signal usually travels through the ECM before going to the fuel gauge, but not always; it depends on your vehicle.
How Accurate Are Fuel Gauges?
If the fuel gauge is functioning properly, it is extremely accurate with a few caveats.
#1. After filling up.
When you fill up the tank, you probably notice it takes a little while for the fuel gauge to move down, and this is related to placement inside the tank.
The float gauge is typically placed in different locations inside the fuel tank. Nearly all float gauges are not placed at the top of the tank, but somewhere below to allow for some space. It takes time for the fuel gauge to detect the gap difference, and after that, the operational gauge should be nearly perfectly accurate.
#2. Under 1/4 tank.
The other time a gauge is not very reliable is when the tank is really low. Many float gauges do not go all the way down, and a float gauge can return an empty reading when there is still some fuel in the tank.
#3. Driving on an incline.
Finally, simply put, anytime you are on an incline, the reading is going to be off. The fuel will settle in the lowest part of the vehicle, but the fuel gauge is going to stay at the same height reading.
If the fuel is sitting near the bottom of the tank, the gauge will reflect as having more fuel than it actually does. If the tank is nearly full, the gauge is used but reflects nearly empty based on the fuel rising higher than the float gauge.
Once you level or portion the vehicle back to level, the gauge reading(s) will recalibrate based on the type of fuel gauge that you have.
What Causes a Fuel Gauge to Malfunction?
When your fuel gauge doesn’t work, there are potential causes and there are symptoms that need to be remedied. We’ll outline three different ways that the fuel gauge can act up, then we’ll look into what might cause each issue.
#1. The Gauge Is Reading Empty When It Is Full.
If you can never get your fuel gauge to read full, chances are you have a faulty fuel gauge or a burnt electrical connection.
Most fuel gauges get full when they are getting maximum power, so with zero power, they’ll read empty.
You could also have a faulty float gauge, too, because if that doesn’t send any signal, then you will read empty. Before delving into anything here, check fuses and power at the gauge before testing further.
#2. Gauge Is Stuck On Full.
If your fuel gauge always reads full, it’s either a faulty gauge that’s stuck on full, or the float gauge is always sending a full signal.
Many times, you can determine which part is faulty just by observing if the fuel gauge resets to empty when you shut the vehicle down.
However, that’s vehicle-dependent because some float gauges lock at their last setting. But if your vehicle normally cycles on and off, and it does not cycle now, then the issue is most likely a gauge in the dashboard.
#3. Gauge Fluctuates Up and Down.
This is the most complex condition to diagnose because there are a few potential causes. You could potentially have a faulty float gauge or a faulty gauge in the dash, but the issue may not be as simple to diagnose.
One possible explanation is that your float gauge may be a bit too loose. When you hit bumps and dips on the road, the fuel may hop up and down, causing your gauge to fluctuate.
It could also be part of a normal condition. For example, if you parked on a hill, it’s completely normal for your fuel reading to change.
That’s because your fuel will change shape based on gravity, and whatever position the float sits will rise or fall. Once you level your vehicle out, you’ll be right back to getting an accurate reading.
How to Fix a Gas Gauge (and Repair Cost)
Repairing a gas gauge will take a little diagnosis, and if you have access to the proper tools, it is not a particularly difficult job to do.
First, rule out the simple items. For this job, that will be the fuses and circuit breakers. Once that is done, we suggest using an OBD-II scan tool. Hook the scan tool up and take a look at the fuel reading.
If your automotive scanner has an accurate fuel reading, your problem lies in the gauge itself and not the float gauge. If it does not have an accurate reading, it is likely in the float gauge.
Although you can check for power at the gauge manually (if you check for power at the gauge and everything is powered up like it should be, replace the gauge), the OBD-II scan tool will eliminate you having to dismantle the dash if you show that the problem is with the float gauge.
If the gauge is the problem and you are unable to check power, you will need to dismantle the dash and check for power at the gauge. If you have power at the gauge, replace the gauge.
If you have a valid OBD-II scan tool reading and the gauge is not getting power, check the electrical connection between the gauge and the ECM; there is likely a break in the electrical connection.
If the scan tool steered you towards the float gauge, repeat the process for there. Chances are, this problem is one of the two gauges, and while you can occasionally have problems with wiring, it is very rarely the issue.
As a summary, we’ve put together a brief chart detailing expected repair costs based on your car’s exact issue.
Be warned, prices can vary greatly depending on what you drive! Some aftermarket float gauges will cost about $50, while others might be $700 or more!
Cost to Fix a Malfunctioning Gas Gauge
| Problem | DIY Cost | Repair Shop Cost |
|---|---|---|
| Faulty Fuse/Circuit Breaker | $1 to $20 | $10 to $40 |
| Faulty Fuel Gauge | $50 to $200 | $250 to $500 |
| Fault Float Gauge | $50 to $700 | $250 to $1000 |
| Electrical Wiring Issue | $30 to $60 | $250 to $450 |