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. Depending on the type of fireplace you are looking at, there is more than one way to obtain this reading. The most common way of obtaining it is to disconnect the thermocouple from the valve by unthreading the thermocouple connector nut with a 3/8" open end wrench.

Next attach the leads of your multi meter to the end positive contact and the copper casing using alligator clips. After ensuring that your multi meter is on the proper range, light the pilot and hold in the control knob until the readings on the multi meter stabilize. This should take about 45 seconds. A reading of 18 to 30 mVDC should be expected. Your reading may be negative or positive depending on the polarity, but this is not important.

Should your reading be low, check your pilot flame before replacing the thermocouple.

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