Gas Valve Diagnostic

Test your furnace gas valve to ensure it's working normally.

Start here

The gas valve controls the flow of gas to the burners. It activates when there is a call for heat and the furnace startup sequence has safely completed.

Refer to the furnace glossary if you need help identifying furnace parts or terms.

Testing the gas valve

Use this procedure to diagnose if the gas valve is the source of your furnace problem — after you’ve done basic troubleshooting and found any of the following to be true:

  • error code indicates “faulty gas valve”
  • during the sequence of operations the igniter glows orange but you do not see flames*

*If you see flames briefly (0.5 – 2 seconds), go to the flame sensor diagnostic.

Note: This diagnostic does not apply to modulating gas valves or “smart” gas valves (found on early mid-efficiency furnaces that have an inducer motor and standing pilot light).

Tools needed

  • multimeter that can measure AC voltage

Testing procedure

  1. Turn off the power to the furnace.
  2. Remove the upper furnace cabinet cover. Leave the lower furnace cabinet cover in place.
  3. Locate the gas valve. You’ll find a pig tail connector at the top or side of the valve with two or three wires leading out of it.
  4. Unplug the connector from the gas valve. If there is a lock on the connector, push it in first.
  5. Set your multimeter to measure AC voltage.
  6. Contact the multimeter probes to the connector you’ve just unplugged:
    • Two-wire connectors: one probe to each pin.
    • Three-wire connectors: one probe to the pin for the white wire and one probe to any other pin.
  7. Ensure the probes stay in contact for the duration of the test. A second person can be helpful for this part.
  8. Turn on the power to the furnace and set the thermostat to call for heat.
  9. Watch the furnace startup sequence. After the igniter glows orange hot for 15-30 seconds, take note of the voltage reading on the multimeter, if any.
    • For three-wire connectors: if you did not measure any voltage at all, leave one probe in contact with the pin for the white wire, but switch the other probe to the third pin. Reset the furnace power and repeat this step.
  10. Turn off the power to the furnace and reconnect the wires to the gas valve.

Interpreting the results

Note: It’s normal for the gas valve voltage reading to fluctuate between 0V and 4-5V during all stages of operation.

Determine which of the following cases best matches your measurement, after the igniter glows for 15-30 seconds:

  • Voltage increases to 24-30V for one to three seconds before the igniter shuts off: Control board is operating normally. Very likely that the gas valve is faulty.
  • Voltage does not change or increases less than 15V: Control board is faulty and should be replaced. Gas valve may or may not be faulty.

Do NOT attempt to replace the gas valve yourself.

Gas valve replacement should only be performed by a qualified and experienced service technician.

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Furnace repairs should only be performed by a certified technician.

This website is for informational purposes only. Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.

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