1. In combination with an electric heater for establishing an elevated temperature above ambient at a heated region spaced therefrom, and a temperature control system including a pair of sensors located adjacent each other at said region, signal comparing circuits respectively connected to said sensors for producing difference signals in response to deviation of the elevated temperature from a predetermined reference value and switching control means operatively connecting one of said signal comparing circuits to the heater for intermittent energization thereof tending to maintain the elevated temperature of the region at said predetermined reference value, the improvement residing in fail-safe assurance means comprising a third sensor located adjacent to the heater, shut-down control means connecting said third sensor to the switching control means for deenergizing and maintaining the heater deenergized in response to an excessive temperature differential between the heater and the heated region, and signal drift detection means connecting the other of the signal comparing circuits to the shut-down control means for alternatively deenergizing and maintaining the heater denergized in response to bidirectional increase in the difference signals beyond a predetermined limit.