An electric preheater for an internal combustion engine includes a heater having a metallic base member adapted to be secured to and cover an opening of an engine block and carrying an electric immersion heating element adapted to project through the opening into the coolant passage of the block. The base member has electric terminals for supplying power to the heating element. An electric power supply cable is detachably connected to the terminals by a molded plug having mating connectors embedded in a projecting resilient boss on the plug. A heat sensor, in the form of a thermistor, is embedded in the boss and has an exposed sloping leading surface at the side of the boss to facilitate insertion of the plug into a hollow recess defined by an encircling metallic side wall provided on the base around the heating element terminals. The heat sensor engages the encircling side wall portion of the base member in good heat exchange relation therewith and monitors the temperature of the heating element and base member to effect automatic control of the energization of the heating element by means of an electronic temperature control circuit.