Low Fuel Light Comes on When Tank Is Full – Causes and Fixes

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.

Low Fuel Light Comes On When Tank Is Full

What Causes My Gas Light On When My Tank Is Full?

To know the cause of this problem, we need first to know how the car measures the amount of fuel. We also need to know how it senses the changes in the fuel tank. This mechanism is called a fuel level sensor. It depends on measuring the voltage across the variable resistor to determine the level of the fuel remaining in the tank. It consists of:

  • Float Switch: It is usually made of foam, and its job is to float over the fuel quantity in the tank to give a physical representation of the fuel amount.
  • Variable Resistor: This is connected to the float via a metal needle to sense the variations in the float height.
  • Wiper: It looks like a very small windscreen wiper, which moves over a resistive material.

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This is how it works? The float switch floats over the fuel’s surface and is connected to the variable resistance with a metal connecting rod. The end of the variable resistor is attached to the ground with the wiper moving over the resistive material. When the fuel level changes, the wiper moves across the variable resistance, causing the voltage to change. Therefore, when the tank is empty, the wiper points to high resistance, and vice versa. When the fuel tank is full, it provides less resistance. Therefore, the fuel gauge can understand this formula as it is preprogrammed on it, and work accordingly

As we all know, this problem is related to the fuel level sensor located inside the fuel tank. The sensor consists of a float that is connected to a variable resistance board. They have the shape of small metal lines that touch the board and send the readings to the fuel gauge in the driver’s dashboard. The metal lines become corroded and dull as they become old, due to the low-quality fuel or any harmful additives added to the fuel

Now, these connected fingers keep making some gaps between the metal-connected sheets and the float needle. They keep on separating and creating gaps, thus resulting in false readings and false fuel levels. At this point, you find the strange behavior of the fuel gauge, which distracts and annoys drivers.

This resistance is connected to the fuel gauge in the driver’s dashboard. The variables’ resistance translates into the wrong fuel readings, making the low fuel light stay on even when the fuel tank is full.

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How to Fix It

Before we diagnose and try to fix the fault, we should be cautious when dealing with any fuel parts to prevent any danger:

First, you have to change or refine the fuel type or grade that you use. This is because the impurities and water (if found) can rust and corrode the variable resistance making it harder to have an accurate reading.

Try not to let the fuel tank out of fuel since this allows the filth from the bottom of the tank to get stuck in the variable resistor and make it hard to give the correct reading.

Using an ohmmeter measure the resistance between the two terminals of variable resistance. You then need to compare it with the full and half tank values stated in the vehicle manual.

If you find another value, this means that the variable resistance is having a problem. Try to rub it with abrasive and rust cleaner and try one more time until you get the pre-stated values. You can then place the sensor back. If you still give the wrong values, the fuel level sensor needs to be changed as soon as possible.

One more reason that may damage the fuel level sensor is if the fuel tank cover has a very tiny ventilation hole. The role of this hole is to drain the overpressure. When this hole is blocked, the fiberglass tank shrinks and bends the metal needle between the float and the variable resistance. This makes the system fail and gives non-accurate readings. If you are noticing the fuel tank shrinking, then get the fuel tank cover changed.

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Frequently Asked Questions (FAQs)

fuel light on with half a tank

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.

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Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

One thought on “Low Fuel Light Comes on When Tank Is Full – Causes and Fixes

  1. Hi Steve!
    Thanks For Your Query.
    According to your problem, I suggest you to check the wiring line of the low fuel gauge.
    If you find that’s alright, check the Low Fuel Level Sensor. Faulty fuel level sensor can cause this. The Fuel Level Sensor is located inside the gas tank.
    Bad electoral connection can also cause this problem.
    Check these items step by step, I hope your problem will be solved.

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