This probe contains a resistive self-heating element that relies on the presence of liquid to keep it cool. Installed at a minimum fluid level in the reservoir, the probe will trigger an alarm if the liquid falls below the minimum fluid level necessary to cool the element or if the sensor wire fails to function properly.
Installed near a minimum desired fluid level, at the top of the radiator, the probe relies on a current flow to determine if there is sufficient fluid in the radiator. Lack of conductive coolant or failure of the probe wire to function normally will trigger an alarm.