Abstract: A low oil sensor especially for application with an internal combustion engine is provided with an over-center spring and diaphragm structure to maintain the sensor in the inactivated mode whenever there is an adequate oil level in an engine sump. Crankcase pulse pressures provide an adequate pressure differential across the diaphragm to move the switch to activate an indicator circuit and to stop the engine. Inadvertent switch operation is prevented by utilization of a check valve and bleed port arrangement. The switch can be reset by an externally operable reset plunger.