Thermocouple Open Circuit Test Procedure

Purpose

This procedure outlines the steps to test a thermocouple's voltage output using a multimeter to determine if it is functioning properly.

Required Tools and Equipment

• Multimeter (set to millivolts DC)

• 3/8” open-end wrench

• Alligator clip leads

Procedure

1. Preparation

   • Ensure the fireplace or appliance is off and cool before starting.

   • Identify the thermocouple connection point on the gas valve.

2. Disconnect the Thermocouple

   • Using a 3/8” open-end wrench, carefully unthread the thermocouple connector nut from the gas valve.

3. Connect the Multimeter

   • Attach one multimeter lead to the positive contact at the end of the thermocouple.

   • Attach the second multimeter lead to the copper casing of the thermocouple using alligator clips.

4. Set the Multimeter

   • Ensure your multimeter is set to the appropriate millivolts DC (mVDC) range.

5. Ignite the Pilot Flame

   • Light the pilot flame and hold in the control knob to maintain the flame.

   • Continue to hold the control knob until the multimeter reading stabilizes (approximately 45 seconds).

6. Observe the Reading

   • A healthy thermocouple should produce a reading of 18 to 30 mVDC.

   • The reading may display as positive or negative, depending on polarity — this is normal and does not indicate a fault.

7. Troubleshooting Low Readings

   • If the reading is below 18 mVDC, inspect the pilot flame:

     • Ensure the pilot flame is properly adjusted and enveloping the thermocouple tip.

     • Clean or adjust the pilot assembly if necessary before replacing the thermocouple.

Test Results

Conclusion

• If the thermocouple voltage falls below the acceptable range and the pilot flame is confirmed to be correct, replace the thermocouple.

• If the voltage is within range, no replacement is required.