On natural vent appliances incorporating a vent safety switch (VSS) as part of the thermocouple circuit, you can use the ECO (energy cut off) or terminal block on the thermocouple as a test point for gathering information. By removing the VSS wire on the pilot assembly side of the terminal block and then touching your meter probe to this point, and with your other on the thermocouple casing, you can obtain the open circuit reading.

The open circuit test will tell you the amount of voltage that the thermocouple is capable of producing without any load, or when it is disconnected.