If you notice that your gas fireplace suddenly goes off and does not come back on, it could indicate the gas fireplace thermocouple should be replaced. Gas fireplaces operate by preheating the gas that will flow to the burner by using a pilot lamp. Once the gas has heated properly, the thermocouple – a bimetal switch that controls fireplace activation whose sides are set for different temperatures -- reaches its proper temperature. The thermocouple unwinds and switches on the fireplace. Thermocouples are rigorously tested and should last for years. Indeed, experts believe a four-year check cycle is all that is needed to make sure things are functioning properly.