The closed circuit test will tell you the voltage drop across the main operator coil. With the switch or thermostat closed, place your meter leads on the TH and TP terminals and measure your drop in mVDC.

As with the thermocouple, the open circuit test will tell us the potential output of the thermopile. While most thermopiles are designed to produce up to 750 mVDC, a minimum reading of 325 mVDC open circuit is generally acceptable as long as the switch is located close to the appliance and connected with properly sized wire.