A Lennox furnace flashing red light is the control board giving you a code: count the flashes between pauses. On a Merit ML180UHE, 3 means the burner didn’t light or lost its flame, 4 a weak flame signal, 7 a limit open too long, 8 the rollout switch, 9 the pressure switch. Signature and Elite furnaces print an E code instead. A repair starts with our $99 diagnostic; a flame sensor cleaning is $95 to $180.
Our Lennox repair page has every Lennox code and what each repair costs.
Merit ML180UHE: the red light codes
| Flashes | What it means | What it usually takes |
|---|---|---|
| 3 | Burner failed to light, or lost flame during heat | Gas supply, igniter or flame sensor |
| 4 | Low flame signal | A flame sensor cleaning |
| 5 | Watchguard: too many failed tries | Igniter, flame sensor or gas valve |
| 7 | A limit open longer than 3 minutes | The filter and airflow first |
| 8 | Rollout switch open | Call: rollout needs a look at the burners and venting |
| 9 | Pressure switch failed to close or opened during heat | Vent, drain, hose or inducer |
| 12 | Flame sensed without the gas valve on | Call |
From Lennox’s ML180UHE unit information (ignition controls 103217 and 107163).
Merit ML296V two-stage: a shorter list
| Flashes | What it means |
|---|---|
| 1 | Flame present with the gas valve off |
| 3 | Pressure switch open with the inducer running |
| 4 | Primary limit, secondary limit or rollout switch open |
| 7 | Lockout: burners fail to light |
From Lennox’s ML296UHV installation instructions. Same number of flashes, different meaning from the ML180: go by your model.
Signature and Elite furnaces print the code
An SLP99V, SL280V or EL296V shows an E and three digits on the board, and an S30 or S40 thermostat shows the same number as an alert: E223 is the pressure switch, E240 a low flame current, E250 the limit, E270 a lockout with no flame. The full table is on the Lennox page.
Which codes you can deal with, and which you can’t
4 flashes and 7 flashes are often things you can start on: a clogged filter trips the limit, and a weak flame signal is usually a dirty sensor ($95 to $180 to clean). 3 and 5 are the furnace failing to light and are worth one reset, no more. 8 is the rollout switch: flame rolled out of the burner area, and that needs a technician to find out why before anyone resets it. Same with 12.
Lennox red light questions people ask us
Why is my Lennox furnace flashing a red light?
It’s a code. Count the flashes between pauses and match them to your model’s table. The diagnostic to find and price the fix is $99.
What does 8 flashes mean on a Lennox furnace?
On a Merit ML180UHE, the rollout switch is open. Don’t keep resetting it; have it looked at.
What does a steady red light mean on a Lennox?
Lennox’s tables are about flash counts. A light that’s solid or off with no heat is worth a call with the model number in hand.
Is it worth fixing my Lennox furnace?
Usually, if it’s a part. If the heat exchanger has failed, or it’s past about 15 years and needs a big repair, a new furnace is $4,285 to $14,040 installed. From about $71 a month at 0% APR for 60 months, the whole price financed, on approved credit. See the Lennox page.
