In this article, we are going to talk about a major problem people experience in their cars. The driver could be driving his car, and surprisingly the low fuel light comes on.
Usually, the driver will stop at the nearest fuel station to get some fuel for their vehicle. To his surprise, he finds that the car only needs half of the tank to get fully refueled.
The question of why the fuel light comes on when the tank is full is a problem many people face during their vehicle lifetime.
Despite how easy this problem may look, it can make the driver feel worried on the road and be afraid to run out of fuel. We are therefore going to break down this problem to know the root of the problem and how to fix it.
What Triggers The Low Fuel Light?
Before we get into what causes the low fuel light to come on when the tank is full, let’s first look at how your car monitors the amount of gas in the tank.
To keep track of how much fuel is in a car, every vehicle has a fuel level sensor. This sensor system is made up of three key components: a float switch, a variable resistor, and a wiper. The float switch is made from foam and it floats over the top of the fuel inside the fuel tank.
Attached to the float switch is a variable resistor, this metal board can sense the height of the fuel and pick up on any variations. There is also a small wiper attached, the purpose of this is to move over the resistive material and across the resistor board.
When the fuel level inside the fuel tank changes, the wiper will move across the variable resistor and the voltage across the metal rod will change.
If your fuel tank is full, the small wiper will point to low resistance and when the tank is empty the wiper will point to high resistance.
The fuel gauge is preprogrammed to know that less resistance equates to a tank full of gas and vice versa.
When the wiper on the sensor indicates high resistance, your low fuel light will ignite on the dashboard. Once you fill your tank back up with gas, the sensor will have less resistance and will alert your fuel gauge accordingly.
What Causes My Gas Light On When My Tank Is Full?
When the fuel gauge falls below one gallon, the light flashes. If the solid light illuminates, it means there is only half a gallon remaining. But what if the low fuel light comes on when the tank is full? It’s obviously a sign of a system malfunction or the use of faulty parts.
#1. Stuck Float.
If the tank is full and the vehicle continues to flash, it could be due to a stuck tank-level float. The float can get stuck due to storing the vehicle with a partially empty tank, particularly in the winter.
While you can keep driving, as the float should be unstuck eventually and start functioning properly, it’s better to pull the fuel pump and clear the float.
In this case, unplug the fuel unit connector and inspect if the light turns off. If it works, it means that the level unit, such as a float or sender, has been damaged and is no longer functioning properly.
But if the light still turns on, possibly the wiring harness or cluster has gone bad.
#2. Fuel Level Sensor.
A car light may stay on when the fuel tank is full due to the faulty fuel level. To avoid this problem, the best recommendation is to use high-grade fuel. Usually, lower-grade fuel contains more impurities or traces of water that can damage the sensor.
Low-quality gasoline can also damage the car’s metal parts, like fuel sensors, thin prongs, and needles. The water in the cheap gasoline can cause corrosion to build up in the metal parts.
#3. Damaged Variable Resistor.
It consists of a metal needle and variable resistance board, and corrosion can dull and wear down the tiny metal lines on the needle. The variable resistor can also get damaged due to the age or use of low-grade fuel additives.
If the lines on the needle rust excessively, it might not be able to function properly to send the correct reading to the fuel gauge. As a result, the low fuel light comes on when the tank is full.
#4. Incorrect Gas Gauge Reading.
Suppose you filled up the tank a week ago and drove regularly. You can’t expect improved fuel efficiency all the time.
The vehicle can get incorrect gas gauge readings due to a problem with the variable resistor. Possibly a faulty open circuit in the resistor, faulty wiring, or a bad gauge is causing the error.
Another reason for the cars getting the wrong gas gauge reading and turning on the fuel light with a full tank is stuck the gauge on empty.
Possibly the floats get disconnected from the metal arms and can’t send the right information about fuel level. A malfunctioning variable sensor or corroded wires in the system can also be the root of the problem.
In addition, if you play a guessing game while attempting to figure out the amount of fuel in the tank, it can cause the float to stick in place and jolt back into motion due to the movement of the vehicle.
It can also introduce irregular readings, and coming out with the low fuel light when the tank is full is the most potential one. Whatever the reason is, you need a technician instead of a DIY fix.
#5. Blocked Fuel tank ventilation hole.
The ventilation hole at the top of a fuel tank can get blocked, which can then shrink the fiberglass tank. It can also bend the metal needle between the variable resistance and the float and disturb the movement across the variable resistance board.
As a result, the sensor may fail and won’t be able to deliver the correct reading to the fuel gauge, and you may experience confusing consequences.
To avoid this hassle, check the fuel level sensor regularly so the tiny hole on the fuel tank’s top doesn’t get blocked due to dirt or debris. If it already happens, you may need to change the fuel tank cover.
How To Stop Fuel Light Coming On When There’s Fuel In The Tank?
Damage to the fuel sensor inside the tank is often the main cause of the low fuel light coming on when it doesn’t need to. The fuel sensor will send readings to the fuel gauge and if the sensor is damaged, the readings will not be correct.
To prevent damaging your car’s fuel level sensor, be sure to do the following:
#1. Use high-quality gasoline.
Cheap gasoline can damage the metal parts of the fuel sensor, in particular, the needle and thin prongs on the variable resistance board. Low-quality gas can also contain water, this can cause rust and corrosion to build upon the metal parts.
#2. Never drive on ’empty’.
If your low fuel light has come on, don’t keep driving for miles on end and hope your car will keep working on sheer willpower alone. When you run your car on the dregs of the gas left in the tank, the fuel at the bottom can resemble sludge.
The thick consistency of the fuel at the bottom of the tank can damage the fuel level sensor as it moves over the surface.
#3. Check the ventilation hole.
Blockages in the ventilation hole at the top of the fuel tank can cause problems for the fuel level sensor. It may be beneficial to regularly check that the small hole at the top of the fuel tank hasn’t become blocked with dirt or built-up debris.
By following the steps above, you should be able to keep your fuel level sensor in full working order. However, this information is no good to you if your sensor is already malfunctioning, but you should keep this advice in mind for the future.
If your low fuel light is coming on when you know there is fuel in the tank, here is a method you can use to try and solve the problem:
Step 1 – Remove the fuel tank assembly. You may need to seek professional advice on how to do this without causing any damage to your vehicle.
Step 2 – Using an Ohmmeter, connect the red and black cables to the two terminals on the top of the fuel tank assembly. You will need to research your vehicle to learn which two terminals you need to connect the ohmmeter to.
Step 3 – Make sure the Ohmmeter is on the correct setting – it should be displaying either ‘Ohm’ or the Greek letter for Omega.
Step 4 – Make sure the fuel sensor float is pushed down and note down the reading that is displayed on the Ohmmeter.
Step 5 – Next, move the float sensor up. This replicates the action that happens when fuel is added to the tank. As you move the sensor up the reading on the Ohmmeter should decrease.
Step 6 – If the two readings are as you would expect, the fault may not be with the fuel level sensor. Your fuel gauge may have an issue that needs a professional diagnostic.
Step 7 – If the reading on the Ohmmeter is the same when the sensor is up and down, there is likely a problem with the sensor and it needs replacing.
You can either replace your sensor at home by following YouTube tutorials or consulting forums for your specific car model, or you can take your car to a mechanic.
Step 8 – If you don’t want to rush to replace the sensor, try cleaning it carefully with a small brush and rust remover. Record the reading again, if there is no difference to your first attempt then a sensor replacement is likely necessary.
Step 9 – This process may sound a little complicated but once you know what you are doing, using the Ohmmeter is not difficult. If you find visual guides more useful, here is a tutorial showing how to fix a fuel level sensor:
Driving without a working fuel tank sensor can be risky. If you don’t know how much fuel is in your tank, you could potentially break down at any moment!
Also, running your car on very little fuel is not good for your car’s health as the engine will be trying to run on the dirt and sludge that has built up at the bottom of the fuel tank.
FAQs.
Q. How Much Fuel Is Left After Light Comes On?
This answer varies depending on the fuel tank’s capacity since it is different from one vehicle model to another.
However, in general, when the fuel gets less than a quarter of the fuel tank capacity, the light comes on. For instance, if your tank capacity is 45, then when the fuel remaining is around 11 liters, the fuel light comes on.
Q. How Do You Reset Your Gas Gauge?
If you feel that your fuel gauge is giving wrong and unreal readings try to reset the fuel gauge. Turn the key to the “ON” position and press the “ODO” button until you reach odo mode.
Turn off the ignition, press and hold “ODO” and turn the ignition switch “ON.” Release the “ODO” button and press it again until the screen writes 1.
This means that the reset process is on, when the odometer returns back to normal, release the button. This means that the reset process is done successfully.
Q. How Long Can You Drive With Empty Gas?
It is not recommendable to drive your car while the fuel lamp is on. Your car may give you 50 miles driving on an empty tank.
This will, however, vary depending on the type of your car, the condition of your vehicle as well as how you dive.
If possible, avoid driving your vehicle with an empty gas since it will cause more damage, which may cost you a fortune to fix.
Q. Is It Bad To Run a Car On Low Fuel?
This has many disadvantages. The first is that the fuel in your tank acts as a coolant for the electric pump motor.
If you are always driving on a low fuel tank, the motor will reach high temperatures and eventually blow up and stop working.
Secondly, at the bottom of the fuel tank, all the impurities and particles are found in the fuel.
Therefore, if you drive on the last fuel liters, these particles will be sucked into the fuel filter resulting in blockage.
Q. How Much Does It Cost To Fix Fuel Gauge?
The price of fixing your fuel gauge will depend on your car’s model and the type of fuel gauge you are using.
The average price for fixing the fuel gauge is $25 to $800 depending on the brand, excluding labor costs and taxes. This cost will also differ depending on where the fixing will be done since people charge labor differently.
Final Words
If your vehicle has any of the above issues of fuel gauge not reading correctly or fuel gauge dropping quickly, you need to get your fuel level sensor and fuel gauge checked by an expert.
Many people do not know that they have a problem when the fuel gauge reads empty when full. Always use a high-grade fuel since it elongates the age of the fuel sensor. You also need to avoid driving your car when the fuel lamp is on.