Fireplace Thermocouples & Thermopiles

Which thermocouple does your fireplace need?

A thermocouple is the copper probe that sits in the pilot flame and holds the gas valve open; when it fails the pilot lights but will not stay lit once you release the knob. Three things decide the part: thermocouple or thermopile (a thermopile is the fatter, higher-output version that also powers the main valve on millivolt systems), thread and length (most are M8 or 11/32" thread, 18" to 36" long), and standard vs fast-dropout (PSE/SIT fast-acting types are required on some Valor, Superior and Astria models). Universal thermocouples fit the majority of millivolt pilots; use the OEM part where the manufacturer specifies an interrupter block or a specific length.

Test before you buy: with the pilot lit, a good thermocouple reads 20 to 30 millivolts across its leads; below about 12 mV it is done. If the pilot stays lit but the burner will not come on, suspect the thermopile (should read 350 to 750 mV) or the valve, not the thermocouple. Full walkthrough: how to replace a gas fireplace thermocouple. Need the whole pilot assembly? See fireplace parts.