An apparatus is provided for controlling the operation of a gas fired appliance having a gas valve circuit for enabling the flow of gas to the burner. The apparatus comprises a sensor capable of detecting the presence of flammable vapor, a switching means in series with the gas valve solenoid circuit for interrupting a thermocouple voltage to the gas valve solenoid to disable the flow of gas, and a fuse that normally conducts a current to the switching means to maintain the switching means in a conductive state for applying a thermocouple voltage to the gas valve solenoid. In response to the detection by the sensor of the presence of a predetermined level of flammable vapor concentration, a second switch supplies a higher current through the fuse. The higher current causes the fuse to blow and interrupt the current for maintaining the switching means in a conductive state, such that the switching means interrupts the thermocouple voltage applied to the gas valve solenoid to shut off the gas to the burner.