Flame Sensor Diagnostic
Test your furnace flame sensor to ensure it's working normally.
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The flame sensor is mounted with the steel rod end directly in the flame path of one of the burners in the burner assembly. It is typically located near the burner port, at the opposite end of the burner box from the igniter.
The spade connector on the end of the flame sensor is connected to the control board with a single wire, which is typically white with a female friction-fit connector.
Refer to the furnace glossary if you need help identifying furnace parts or terms.
Testing the flame sensor
Use this procedure to diagnose if the flame sensor (also known as flame rod) is the source of your furnace problem — after you’ve done basic troubleshooting.
Tools needed
- standard multimeter with setting to measure DC micro amps ‘μA’
- multimeter leads with clamps (helpful but not required)
Testing procedure
- Turn off the power to the furnace.
- Remove the upper cabinet cover, but leave the lower cabinet cover in place.
- Locate the flame sensor.
- Gently disconnect the wire from the end of the flame sensor.
- Set your multimeter to measure DC micro amps ‘μA’.
- Connect one multimeter lead to the exposed metal connector on the flame sensor.
- Connect the other lead to the wire that you disconnected, using the metal part of the female connector to ensure a good connection.
- Ensure that the exposed metal portions of the wire and leads are not touching any steel surfaces inside the furnace (to prevent them from grounding out).
- Turn on the power to the furnace.
- Turn the thermostat to ‘heat’ mode and initiate a call for heat.
- Watch the furnace go through normal sequence of operations. When the igniter comes on and gas valve opens the gas will ignite to create a flame.
- When you see the flame, watch the reading on your multimeter. It should increase from 0 to between 0.1 – 5.0 μA. This is the current that the flame sensor is sending to the control board. Record this reading for later.
- Turn off the power to the furnace.
- Disconnect the multimeter.
- Reconnect the wire to the flame sensor.
Interpreting the results
Determine which of the following cases matches your result:
Multimeter reading 2.0 μA or more
If the reading is 2.0 μA or more and you see flames for more than five seconds, the furnace recognized that the gas ignited safely and will continue to run.
The flame sensor is working properly and is probably not the issue.
If flames continue to cut out after a short time, the burner assembly could be dirty or the control board relay may be faulty. Contact a professional technician to diagnose the issue.
Multimeter reading less than 2.0 μA
If the reading is less than 2.0 μA and you see flames for less than five seconds, the furnace did not recognize that the gas ignited safely, so it automatically shut off the gas valve.
The flame sensor is dirty and should be replaced. It is either the primary issue or contributing to the problem.
Note: In some cases, the reading will be less than 2.0 μA but the flames will stay on and the furnace will continue to run. Because most furnaces require at least 1.5 μA of current from the flame sensor to consistently recognize gas ignition, the furnace may startup intermittently.
Why replace the flame sensor instead of cleaning it?
Some furnace technicians simply remove and clean the flame sensor with steel wool or a scouring pad, which can solve the issue in the short term.
We’ve found that the best practice is to replace the flame sensor. Constant use and age can wear down a flame sensor and make it unreliable, even with cleaning.
The good news is that flame sensor replacement is one of the easiest and least-expensive furnace repairs you can do.
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