Thermocouple

What is a Thermocouple?

A thermocouple, is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage. This voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature or flame sensors.

How to test?

Standard Gas Millivolt Thermocouple

Disconnect the thermocouple from the gas valve
using a multi-meter with clamp leads
place the red (+) clamp on the outer (Copper) of the thermocouple, anywhere along its length
connect the black (-) clamp to the end of the thermocouple, which has been removed from the gas valve
put your multi-meter on to mV DC (Millivolt Direct Current), this may show as the ⎓ symbol. You may need to change the range on your multi-meter for mV.
with this all connected, ignite the burner or pilot of the appliance, keeping any buttons pressed in to keep the flame lit on to the thermocouple.
with the thermocouple still in the flame, check the mV displayed on your multi-meter.

For a millivolt gas valve, such as a Robertshaw, you should expect a voltage of over 20mV for the gas valve to work. For other controls, such as gas taps a voltage of about 15mV would be expected.