Top 3 Reasons Why Low Fuel Light Comes on When The Tank is Full

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We all want to avoid our car running out of fuel. That’s why modern cars have a dashboard warning light and symbols that flashes when there isn’t much left in the tank to prevent it from happening. However, what if the low fuel light comes on even though you have enough fuel in your tank?

Low Fuel Light Comes on When The Tank is Full

In that case, you can become anxious while driving, worrying that something is not right, and it will probably distract you from driving. As a response, we will analyze this low fuel light issue to determine its cause and the best course of action.

1. What’s wrong with my fuel gauge?

This article explains why low fuel light comes on when the tank is full. In some cases, low fuel light sometimes comes on even if the tank is half-full or quarter-full. Here are the top 3 common reasons why you got the wrong fuel level:

  1. Blocked ventilation hole
  2. Faulty fuel level sensor
  3. Faulty fuel gauge

2. How does the fuel level sensor work?

In March 2022, Audi recalled roughly 46,000 vehicles to fix inaccurate fuel readings caused by malfunctioning fuel level sensors in their gas tanks. This puts the importance of fuel level sensors for cars into perspective. The fuel level sensor is one of at least 30 sensors a typical car has. 

This sensor uses voltage measurements across variable resistors to estimate how much fuel is still in the tank. The sensor’s mechanism starts with the float switch, which floats above the fuel’s surface and is connected to the variable resistance via a metal connecting rod.

Then, the wiper moves across the resistive substance while the variable resistor’s end is attached to the surface. The wiper glides across the variable resistance when the fuel level fluctuates, changing the voltage. As a result, the wiper indicates high resistance when the tank is empty and vice versa. 

When the gasoline tank is full, it reduces the resistance. Consequently, since this formula is preprogrammed into the gasoline gauge, it can comprehend and function as intended.

3. What causes the low fuel light to come on when the tank is full?

The fuel level sensor may experience issues if there is a blocked ventilation hole at the top of the fuel tank. This is the most likely cause of the low fuel light appearing when the tank is full. Frequently checking that the little hole at the top of the fuel tank is not clogged with dirt or accumulated debris may be helpful.

How to fix it? 

Checking the vent on your fuel tank requires caution to avoid damaging the tank or letting gas escape into the air.

  1. Locate the air vent pipe first, and then insert a compressor. Air it up a little. Unblocked lines will make a gurgling noise. If the line is blocked, you will hear no sound. 
  2. Disconnect the canister vent line from the fuel tank and blow through it to further examine the issue. 
  3. Next, test it with pressurized air. Many of them have long plastic tubes that may be used to blow air through them and clear clogs. 
  4. To check for obstructions, try passing plastic tubing through the vent. 
  5. Finally, you must empty the tank and allow the interior to breathe. Attach the vent hose and fuel line once it has dried, then use the compressed air to try another air blow. Now, there shouldn’t be an obstruction.

4. What causes the low fuel light to come on when the tank is half-full?

The low fuel light frequently illuminates when it shouldn’t be due to a malfunctioning fuel level sensor. Over time, the small metal lines on the needle can become dull and wear away due to corrosion. 

Suppose the lines on the needle corrode too much. In that case, they may be unable to touch the resistance board, especially if the tank is half-full. Consequently, it may result in inaccurate readings on the fuel gauge.

How to fix it?

  1. Carefully remove any broken wires from the variable resistor and replace them with new ones. 
  2. Buy the right-sized wires; else, attaching them to the resistor would be challenging. 
  3. Clean the sensor’s components thoroughly and remove any accumulated filth before attaching the new wires. 
  4. After the sensor has been repaired, you must test it with an ohmmeter to ensure everything is operating.

5. What causes the low fuel light to come on when the tank is quarter-full?

The sensor in the fuel tank is preprogrammed to provide readings to the fuel gauge. If your fuel tank is only quarter-full, you can typically see a problem with the fuel gauge itself rather than the sensor in the fuel tank. Here’s a video explaining how to troubleshoot the fuel gauge and sending unit:

How to fix it?

Simple tools, plenty of patience, and the following instructions are all needed to fix a fuel gauge.

  1. First, begin by repeatedly turning the ignition on and off. Examine the gas gauge’s needle and see if it moves. If not, you most likely have a blown fuse that has to be replaced. 
  2. You can find the fusebox under the driver’s side dash or in the engine compartment. To find the fuse’s precise placement and replace it, it is best to consult the owner’s manual. 
  3. Test the grounding wire on the fuel tank’s transmitting unit after that. In the long term, this fuel component of your car is crucial since if you accidentally leave the tank empty, it may impact nearby components. 
  4. To check the grounding wire on the fuel tank’s sending unit, connect the negative jumper cable to the vehicle’s chassis and the positive jumper cable to the grounding terminal of the sending unit.
  5. If the fuel gauge functions when the jumper wires are connected, check to see if the grounding wire on the sending unit needs to be replaced.
  6. Disconnect the connections from the gas sending device, and then check the fuel gauge. If the needle is pointing out on the empty reading, you have a bad fuel gauge. 
  7. Swap out the fuel gauge to make it right. On the other hand, if the needle is pointing out on the full reading, the transmitting unit is broken and needs to be replaced to fix the gauge. 
  8. Last but not least, make sure that all of the cables are firmly attached to the gas gauge’s rear. Remove the dashboard to gain access to the instrument panel.
  9. Connect the negative jumper cable to the vehicle’s chassis and the positive jumper cable to the fuel gauge’s grounding terminal. You may check the grounding wire on the fuel gauge. 
  10. Start the engine and then examine the fuel gauge. You will need to change the grounding wire if it isn’t working.

6. Key takeaways

After just fueling your car, it can be frustrating and confusing to see the fuel light come on. This issue is frequently caused by problems with the fuel level sensor, fuel gauge, or ventilation hole in your car’s fuel tank. 

In this post, we have explained why your low fuel light is on even when your tank is full, half-full, or quarter-full and given you a few solutions. We hope that this information will be helpful and that you can quickly fix the sensor to get rid of the inconvenient fuel light.

7. What’s next?

Suppose your car is having any of the problems mentioned earlier. In that case, you can perform quick tests to determine whether the problem is with your fuel level sensor, fuel gauge, or ventilation hole. 

We advised running all the tests before replacing a sending unit that may still be working. Sometimes the solution is as straightforward as replacing the grounding wire or the fuel pump. You can read the article linked for additional details on common issues after changing the fuel pump.

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