Abstract: A fail-safe thermostat for vehicular cooling systems is positioned in a junction leading to an internal coolant passageway, a radiator passageway and a bypass passageway. An open housing seated in the junction supports a fixed central rod extending into the wax filled interior of a movable cylindrical member which controls the position of the interior segment of a main valve. Expansion of the wax forces the cylindrical member in a first direction against a compression spring. Fusible alloy is disposed in the coupling between the cylindrical member and the main valve, and melts at a specific temperature range in excess of the normal range. When this occurs the coupling is released in such a way that the spring forces the valve member to a second limit position opposite to the direction of expansion. A bypass closure member at one end of the cylindrical member may be used to close the bypass passageway concurrently with increased temperature.