Float switch
The floating sensor that turns a sump pump on as water rises.
A float switch is the buoyant trigger in a sump pit (or some water heaters and HVAC drain pans). As water rises, the float lifts and switches the pump on; as the water drops, it shuts off. Floats get stuck against the pit wall or jammed by debris, which is the most common reason a sump pump fails to run. Check that yours moves freely when you test the pump.