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Your Fuel Level Sensor (Fuel Tank Sender Unit) worn out? » Read more about common malfunctions • Repair • Replacement manual
Always an eye on the level with the fuel level sensor
The fuel level gauge is a sensor, providing the driver with information. As a rule it is separably installed in the fuel tank. A reliable fuel level sensor ensures safe driving and precise indication of the car's actual driving range.
Fuel level sensor: construction and function
The fuel level sensor is an electromechanical part, its chief component being the float and a connected potentiometer. The float can be connected to a lever arm, moving in an arc. In this case the potentiometer measures the angle of the lever arm. Another, thinner version consists of a central tube with the float directly attached to it. In this version the movement of the float is linear. The moving float causes a change of the resistance in the potentiometer. This change is processed into a signal and transmitted to the control unit and the fuel level indicator. Modern cars have a digital fuel level indicator with integrated range calculator. In many cases, the fuel level sensor is connected to the pre-feed pump in the fuel tank. In this configuration, replacement is difficult and often impossible.
Fuel level gauge defects
Defects of the fuel level sensor can be mechanical or electronic. The potentiometer can be worn or the contacts of the signal line corroded. These malfunctions are rare. More frequently the float is jammed or broken off. All in all, this does not happen very often either. The fuel level sensor usually lasts at least 150.000 km or longer. Its replacement is less easy than generally assumed.
Symptoms and causes for improbable tank level values
The fuel level sensor is integrated in the fuel tank. Depending on the vehicle, several sensors can be installed. It is therefore important to properly assess the defect, drawing the right conclusions. Typical symptoms are:
- The indicator continually showing the same value: either "full", "half full" or "empty".
- The indicator erratically jumping
- The range indicator showing implausible values.
A faulty indicator is not necessarily the result of a defective fuel level gauge. A malfunctioning fuel pump, a leakage of the tank or a clogged tank ventilation can affect the the indicator. In case of an erratic, "jumping" indicator the signal line is often damaged. Cleaning the contacts may offer a solution. This malfunction sometimes occurs in older cars with manual gear. For modern digital tank level indicators this malfunction is rare. In case of a "hanging" level indicator, the float is jammed. The only solution is removal of the fuel level gauge and making it operable again. This malfunction occurs after regular use of inferior quality fuel. This is not likely to happen in Western Europe. Alternatively, a mechanical defect can cause this malfunction. When the range indicator drops quickly, the first hunch is a leakage in the fuel line or the fuel pump of the engine. Most reliable is a smelling test: if the car smells strongly of fuel, this is not a fuel level sensor malfunction. The car should no longer be driven, as a fire hazard is imminent.
Fuel level sensor repair
In order to gain access to the fuel level sensor, the tank must be uncovered first. The access is usually behind the back seat or under a cover in the boot. The operation manual contains the specifics. Generally, the fuel level sensor must be separated from the wiring and unscrewed, then it can be pulled out upwards. As soon as it is removed, the gasket should be replaced. Often, the fuel level sensor cannot be removed. In this case the only option is replacement of the entire tank. This makes the repair considerably more complicated and expensive, nevertheless it can be done by an experienced DIY'er.
Fuel level sensor costs
The price of a new fuel level sensor starts at 8 EUR (± £7). If it is combined with a pre-feed pump, it can cost 80 EUR or more (± £70). It is nevertheless recommended to prefer branded quality, e.g. BOSCH. This ensures a lasting repair.