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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 .

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.

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.

Both LEDs flashing slowly, alternating

Lockout, burners failed to ignite

Faulty flame sensor, gas valve, or control board.

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

Faulty flame sensor, igniter, or gas valve.

Both LEDs flashing quickly, alternating 

Improper voltage or ground

Faulty wiring, fuse, or igniter.

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:

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.

2 flashes

Closed pressure switch with inducer off

Faulty pressure switch or control board.

3 flashes

Open pressure switch with inducer on

Faulty inducer motor, or blocked condensate trap or intake/exhaust.

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. Professional diagnosis required.

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

Reset furnace power and check for a different code before lockout. Likely a faulty flame sensor.

9 flashes

Line voltage polarity incorrect

Faulty wiring or control board.

On this page:

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.

Repair Wiring or Igniter

$199 or $379 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Flame Sensor, Gas Valve, or Control Board

$199 or $899 to $1399 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Blockage, Pressure Switch, or Inducer Motor

$199, $379, or $899 to $1399 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Flame Sensor

$199 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Wiring or Control Board

$199 or $899 to $1399 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Pressure Switch or Control Board

$379 or $899 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Flame Sensor, Igniter, or Gas Valve

$199, $379, or $899 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Gas Valve or Control Board

$899 to $1399 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Professional Diagnosis Required

$109.95 regular hours call-out & diagnosis fee

Our technician will identify the problem and tell you how much it will cost to repair. Once you know the problem and the cost, you can decide if you want to proceed.

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day