Lennox 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.
G40, G43, G50, G60, G61, 80MGF series 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.
There are two LEDs on the control board that flash in different sequences. The LEDs are typically labelled “DS1” and “DS2” and may be red, green, or one of each colour.
Watch carefully, make note of the pattern, and match it up with the corresponding code below:
1st LED off, 2nd LED flashing slowly
Pressure switch failed to close
Faulty pressure switch, inducer motor, or blocked intake/exhaust.
Both LEDs flashing slowly, alternating
Lockout, burners failed to ignite
1st LED flashing slowly, 2nd LED off
Flame sensed with gas valve off
Faulty gas valve or control board.
1st LED on, 2nd LED flashing slowly
Rollout switch open
Cracked primary heat exchanger or plugged secondary heat exchanger. Professional diagnosis required.
1st LED flashing quickly, 2nd LED flashing slowly
Line voltage polarity incorrect
Faulty wiring or control board.
1st LED flashing slowly, 2nd LED flashing quickly
Low flame sensor
Both LEDs on, not flashing
Circuit board self-check failure or ignition/blower control is wired incorrectly
Faulty wiring or control board failure.
1st LED on, 2nd LED off
Faulty wiring or control board failure.
1st LED off, 2nd LED on
Faulty wiring or control board failure.
ML, EL series 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, which may repeat in a loop. Watch it carefully, make note of the sequence, and match it up with the corresponding code below:
3 flashes
Open pressure switch with inducer on
Faulty inducer motor, or blocked condensate trap or intake/exhaust.
5 flashes
Roll out switch open
Possible faulty inducer motor, blocked exhaust or secondary heat exchanger. Professional diagnosis required.
8 flashes
Burners lost flame too many times, lockout
Reset furnace power and check for a different code before lockout. Likely a faulty flame sensor.
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Furnace repairs should only be performed by a certified technician.
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