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:
Both LEDs off
No power. Continue to the Power diagnostic →
Both LEDs flashing slowly
Power on, no call for heat. Continue to the Thermostat diagnostic →
Both LEDs flashing quickly
Normal operation, call for heat. Continue to the Secondary diagnostic →
1st LED flashing slowly, 2nd LED on
Open temp limit switch. Continue to the Secondary diagnostic →
1st LED off, 2nd LED flashing slowly
Pressure switch failed to close. Faulty pressure switch, inducer motor, or blocked intake/exhaust. Book a professional repair or continue to the Inducer motor or Intake and exhaust diagnostic →
Both LEDs flashing slowly, alternating
Lockout, burners failed to ignite. Faulty flame sensor, gas valve, or control board. Book a professional repair or continue to the Flame sensor or Gas valve diagnostic →
1st LED flashing slowly, 2nd LED off
Flame sensed with gas valve off. Faulty gas valve or control board. Book a professional repair or continue to the Gas valve diagnostic →
1st LED on, 2nd LED flashing slowly
Roll out switch open. Cracked primary heat exchanger or plugged secondary heat exchanger. Book a professional repair or continue to the Roll out switch diagnostic →
1st LED flashing quickly, 2nd LED flashing slowly
Line voltage polarity incorrect. Faulty wiring or control board. Book a professional repair.
1st LED flashing slowly, 2nd LED flashing quickly
Low flame sensor. Faulty flame sensor, igniter, or gas valve. Book a professional repair or continue to the Flame sensor, Igniter, or Gas valve diagnostic →
Both LEDs flashing quickly, alternating
Improper voltage or ground. Faulty wiring, fuse, or igniter. Book a professional repair or continue to the Igniter diagnostic →
Both LEDs on, not flashing
Circuit board self-check failure or ignition/blower control is wired incorrectly. Book a professional repair.
1st LED on, 2nd LED off
Faulty wiring or control board failure. Book a professional repair.
1st LED off, 2nd LED on
Faulty wiring or control board failure. Book a professional repair.
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:
LED off
No power. Continue to the Power diagnostic →
LED on, not flashing
Normal operation. Continue to the Secondary diagnostic →
1 flash
Flame sensed with gas valve off. Faulty gas valve or control board. Book a professional repair or continue to the Gas valve diagnostic →
2 flashes
Closed pressure switch with inducer off. Faulty pressure switch or control board. Book a professional repair.
3 flashes
Open pressure switch with inducer on. Continue to the Secondary diagnostic →
4 flashes
Primary limit switch open. Continue to the Secondary diagnostic →
5 flashes
Roll out switch open. Possible faulty inducer motor, blocked exhaust or secondary heat exchanger. Book a professional repair or continue to the Roll out switch diagnostic →
6 flashes
Pressure switch system lockout. Continue to the Secondary diagnostic →
7 flashes
Burners failed to ignite, lockout. Continue to the Secondary diagnostic →
8 flashes
Burners lost flame too many times, lockout. Likely a faulty flame sensor. Reset furnace power and check for a different code before lockout. If code remains the same, book a professional repair or continue to the Flame sensor diagnostic →
9 flashes
Line voltage polarity incorrect. Faulty wiring or control board failure. Book a professional repair.
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