The fuel level sensor is part of the fuel sender unit and functions to measure the level of fuel in the fuel tank. Its purpose is to provide the driver with accurate fuel level read outs to help prevent the car running out of fuel.

Usually attached to the fuel pump, and normally located in the fuel tank, the fuel level sensor measures the level of fuel in the tank. The fuel level data is sent to the car's fuel gauge, normally located on the dash board. The fuel gauge shows the level of fuel in the tank. The fuel level sensor is usually made up of a float which is connected to a metal rod, which is attached to a variable resistor (an electrical apparatus designed to counteract or resist electrical current).  As the fuel is used, the float lowers in the fuel sender unit. As the float lowers it runs along a moving contact on the resistor which increases the resistance of the float. The fuel level sensor detects the resistance levels and the fuel gauge reflects this by slowly moving down. Instead of this resistive type of fuel sensor, some modern fuel level sensors are capacitive level measurement sensors. These are a more accurate electronic or magnetic type of sensor with no moving parts which function to provide precise fuel level measurement and enable reliable fuel gauge readout. 

Ultimately, the fuel level sensor, with the help of the fuel gauge, enables the driver to see how much fuel is left in the tank. The driver uses this information to fill the car up with fuel as and when required. It helps the driver to make sure the fuel levels don't get too low. The worst case scenario is running out of fuel. Running out of fuel can damage the engine. Running a car on low levels of fuel can also be harmful to engine components and the fuel system, including the fuel filter and fuel pump.

Problems with the fuel system should be fixed as soon as possible. If the fuel sensor is faulty this can affect the accuracy of the information displayed on the fuel gauge and could lead to the car running out of fuel. When replacing this part, it is vital that the replacement part is compatible with the car. Specifically designed for the make and model of your vehicle, the fuel level sensor is the ideal choice.