A switch state assurance mechanism of a power stealing circuit for connection to a thermostatically controlled or other kind of operational system. The mechanism may be an interface for the power stealing circuit to the operational system. A switch or relay of the mechanism may have several states but have an appropriate state for a given status of the power stealing circuit and the operational system. The switch or relay may have a latching characteristic relative to its states. The mechanism may have a detection circuit and a drive circuit connected to the relay, and a control circuit connected to the detection circuit and the drive circuit. The mechanism may check the relay state and change the relay to a desired state. The mechanism may be incorporated in other circuits beside a power stealing circuit.