Igniter Diagnostic

Test your furnace igniter to ensure it's working normally.

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The igniter is made of heat-resistant metal that glows orange hot when current is passed through it. It ignites the natural gas as it is released by the gas valve, replacing the function of the pilot light found on older furnaces.

The igniter is mounted next to the burners on either side of the furnace, typically opposite the flame sensor. It receives power from the control board through two ~3” wires, usually black and white, that end in a male/female connector.

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

Testing the igniter

Use this procedure to diagnose if the igniter or the control board is the source of your furnace problem — after you’ve done basic troubleshooting and determined that the igniter is not coming on when it should.

Tools needed

  • multimeter that can measure AC current (amps) and resistance (ohms)

Testing procedure

  1. Turn off the power to the furnace.
  2. Remove the upper furnace cabinet door. Leave the lower furnace cabinet door in place.
  3. Locate the igniter and disconnect it. It is usually a friction-fit male/female connector that unplugs with medium force. Take care to not damage the connector or wires.
  4. Set your multimeter to measure resistance (‘ohms’ or ‘Ω’).
  5. On the igniter connector, contact one multimeter probe to each of the two pins in the end of the connector.
  6. Ensure that the probes do not touch each other and that you are measuring the igniter itself, not the wires connecting to the control board.
  7. Check your multimeter for the resistance measurement and make note of it.

Interpreting the results

Determine which of the following cases matches your result. Measurements may vary from from no continuity (infinite resistance), 40 – 120 ohms, or several thousand ohms.

40 – 100 ohms

This is the normal range. The igniter appears OK and is not likely to be the issue. Go to the next section to test if the control board is delivering proper voltage to the igniter.

No continuity or infinite resistance

Current can’t pass through the igniter, which means there is a break inside and you should replace it.

Very high resistance (more than 100 ohms)

If you measure very high resistance, there is something else wrong with the igniter and you should replace it.

Zero or low resistance (less than 20 ohms)

The igniter is shorted and needs to be replaced. If it’s a “dead short” the control board could have been damaged. Go to the next section to test if the control board is delivering proper voltage to the igniter.

Testing the control board igniter relay

Use this procedure to diagnose if the control board is the delivering proper voltage to the igniter — only do this after you’ve tested the igniter as described above.

Tools needed

  • multimeter that can measure AC voltage

Testing procedure

  1. Follow steps 1-3 above to locate and disconnect the igniter.
  2. Set your multimeter to measure AC voltage.
  3. Locate the wires that supply power from the control board to the igniter. Contact one multimeter probe to each of the two female pins in the end of the connector.
  4. Ensure that the probes do not touch each other or ground out to any part of the furnace.
  5. Turn on the power to the furnace and set the thermostat to call for heat.
  6. Watch the furnace go through the normal sequence of operations.
  7. After the inducer motor runs for 30-90 seconds, listen for the faint click of the igniter relay on the control board.
  8. Watch the reading on your multimeter. It should increase to 80-120V and hold for 15-30 seconds as the control board provides power to the igniter.
  9. Listen to hear the gas valve click open as it is energized. After one or two seconds it will click close and you should smell natural gas.

Interpreting the results

Determine which of the following cases matches your result.

80-120V measured, gas valve opens (gas smell present)

The control board is OK. The igniter is faulty or not connected properly.

No voltage measured, gas valve opens (gas smell present)

The control board is faulty. The igniter may also be faulty, but start with the control board.

No voltage measured, gas valve does not open (no gas smell present)

The sequence of operations is not reaching the point where the control board energizes the igniter and gas valve. It’s likely there is a pressure switch or inducer motor problem.

In this case, reconnect the igniter and let the furnace cycle multiple times until it shows an error code. Look up the error code to continue diagnosing the problem.

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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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