Common Furnace Error Codes
Look up error codes to identify furnace problems and estimate repair costs.
Locating the furnace error code
Refer to the furnace glossary if you need help identifying furnace parts or terms.
All high-efficiency furnaces and most mid-efficiency furnaces show error codes on the control board, either as a flashing LED sequence or on a display .
Where to find the control board and inspection port
The furnace control board is almost always in the lower compartment. You can view the error code LED or display through a small, clear plastic inspection port in the centre of the lower furnace cover.
If the control board isn’t in the lower compartment, it’s in the upper compartment.
How to access the inspection port
Furnaces with upper and lower furnace covers — The inspection port is usually on the lower furnace cover and is accessible without removing the cover.
Furnaces with a one-piece front cover — Remove the front cover first. The inspection port is usually on the secondary cover for the lower compartment.
If the control board is in the upper compartment — You need to remove the upper furnace cover. If there are thumb screws, loosen them and pull the cover out from the screw location. If there are no screws, slide the cover straight up, then pull it out.
Error codes
Don’t turn off the power to the furnace while troubleshooting — unless instructed otherwise. Turning off the power clears the current error code.
Observe the flashing LED sequence or display code, which may repeat in a loop. Watch it carefully, make note of the sequence or digits, and look it up in your furnace manual or manufacturer’s website.
Find the matching description below to continue troubleshooting and estimate repair costs:
No power
Continue to the Power diagnostic →
Normal operation
Continue to the Secondary diagnostic →
Abnormal or weak flame sensor signal
Circuit board failure or control wired incorrectly
Possible faulty wiring or control board.
Flame sensed with gas valve off
Possible faulty gas valve or control board. Professional diagnosis required.
Gas heating lock out
Continue to the Secondary diagnostic →
Gas valve circuit error
Faulty gas valve or control board.
Ignition lock out
Continue to the Secondary diagnostic →
Ignition proving error
Continue to the Secondary diagnostic →
Igniter fault
Faulty wiring or igniter.
Improper main ground or less than 75 volts
Limit or flame roll out switch open
Continue to the Secondary diagnostic →
Pressure switch closed when inducer off
Faulty pressure switch or control board.
Pressure switch did not close
Continue to the Secondary diagnostic →
Pressure switch did not open (primary or secondary)
Faulty pressure switch, inducer motor, or blocked intake/exhaust.
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Furnace repairs should only be performed by a certified technician.
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