Coolant Level Sensor

It is often a float switch. It lights a warning. Some use electrodes. A sensor can stick and warn falsely, but a warning should always be checked first. Adding the correct coolant type is important. A low level can mean a leak, a failing head gasket or a faulty cap. Checking the level cold against the markings on the tank confirms whether the warning is real.

A float moves with the coolant level.

Check for leaks when it warns. Do not open a hot tank.