The tank sensor.

The tank sensors are as good as all defective.
Similarly those of my little red friend.
The first question is how does it work the second how do we get to components.

The operation is simple in the tank element is a NTC resistance.
This resistance has the property if it gets hot, the resistance value is less.

If we put this resistance in series with a lamp and connect it to 6 volt after some time the light is turned on.
See also the photo on the right.
By the current flowing through the resistance it will be heated up and the lamp will burn.
If we hang this resistance in the gasoline it cools down and then the light goes off and stays off.
Until the tank level is below the level of the resistance, the resistance isn't longer cooled the light will burn again.

This NTC NTC-thermistor is amongst others delivered by Conrad K164 100 Ω Epcos K164 HEISSLEITER 100 OHM.
Please note that installation is done correctly.
he operation is reasonable but greatly depends on how it is mounted in the element.