The short answer
Most Lennox furnaces in Northern Virginia have no green light on the control board at all. The one Lennox furnace with a green status light is the ML296, where a flashing green light means the furnace wants high heat and the high pressure switch hasn’t closed, which is a venting problem. On most other Merit furnaces, a slow, steady blink is the normal heartbeat, not a fault. A blinking green light near the furnace may be the iComfort S30 Smart Hub waiting for a phone to connect.
First, find which light you’re looking at
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| Where the light is | Model | What it means |
|---|---|---|
| Green light on the furnace control board | ML296UHV | Off: no call for high heat. On: high heat, running normally. Flashing: high heat called, high pressure switch not closed. |
| Amber light, same board | ML296UHV | Counts airflow. Each flash is 100 CFM, so 5 flashes is 500. |
| Red light, same board | ML296UHV | Steady on is normal. A flash count is a fault code. |
| One light, slow steady blink (1 second on, 1 off) | ML180UHE | The heartbeat. Normal: idle, fan only, or cooling. |
| Same light, fast steady blink | ML180UHE | Heating. Normal. |
| A dot blinking on a small number display | SL280, EL296, SLP99 | Normal idle. A fault shows as E and three digits. |
| A light on a small box near the furnace, blinking green | iComfort S30 Smart Hub | Waiting for a phone to connect, during setup. Its status light solid green means no errors. |
The ML180UHE’s board light is red, but a slow heartbeat blinking all day gets called every color in the book. If your model isn’t in the table, read the model number off the sticker inside the furnace door and match it.
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The ML296’s green light, flashing
The ML296 is a two-stage furnace. On a cold night it calls for high heat, the inducer fan speeds up, and a second pressure switch is supposed to close to prove the vent can handle the extra exhaust. The green light flashing means that switch hasn’t closed. Lennox lists it as “high heat demand, high pressure switch not closed.”
The usual reasons are the same ones that trip any pressure switch: a vent pipe partly blocked by snow, leaves or a nest, water sitting in the switch’s tubing, a clogged condensate drain, or a tired inducer motor. Check the vent pipes outside and look for water near the furnace. If both are clear, call. Lennox pressure switch codes goes deeper.
The heartbeat: blinking is normal
On an ML180UHE, Lennox calls the slow blink (1 second on, 1 second off) the heartbeat. It means the board has power and nothing’s wrong: idle, fan only, or cooling. A fast, steady blink means it’s heating. A fault looks different: a group of quick flashes, a pause of a few seconds, then the same group again. Count one group and look it up in Lennox flashing red light.
Two more readings worth knowing. No light at all means no power to the board, or a board fault. And on the newer ML180UHE board, a light that stays solid on means the board found a fault.
What to check
- Read the model number off the sticker inside the furnace door.
- Take a short video of the light. Counting flashes off a video beats counting them crouched in a closet.
- Furnace switch on, breaker on, blower door on and seated.
- Outside, the vent pipes are clear of snow, leaves and nests.
- No water around the furnace (condensate drain clogs).
- Thermostat on Heat, set a few degrees warmer than the room.
For any brand, furnace blinking light walks through the common blink codes.
When the blinking is a real fault
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| What we usually find | Light you’d see | Our price |
|---|---|---|
| Clogged condensate drain | Green flashing on an ML296, or a pressure switch count | $175 to $420 |
| Blocked vent or water in the switch tubing | Green flashing on an ML296 | Priced in writing |
| Cracked igniter | A no-ignition count | $240 to $410 |
| Dirty flame sensor | A flame-loss count | $95 to $180 |
| Control board | Solid on (ML180UHE newer board), or no light with power | $550 to $1,180 |
The diagnostic is a flat $99, and the price comes in writing before we start.
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade.
Every Lennox code in one place: Lennox error codes. Other brands too: furnace and AC error codes.
Call or book a Lennox repair
Call (703) 997-9757. We answer day or night. Or book online and pick a time. The diagnostic is a flat $99, and you see the repair price in writing before we touch anything. Furnace running fine and you want to keep it that way? Book the $67 heating check from goswiftpro.com/cookiesoffer (book by November 30) and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276. More on Lennox repair in Northern Virginia.
Questions people ask
Why is the green light blinking on my Lennox furnace?
On an ML296, flashing green means the furnace wants high heat and the high pressure switch hasn’t closed, usually a vent or drain problem. Most other Lennox furnaces have no green light; a slow, steady blink on those is the normal heartbeat.
What does a slow blinking light on a Lennox furnace mean?
On an ML180UHE, a slow one-second blink is the heartbeat: the board is powered and idle, running the fan or cooling. A fast steady blink means it’s heating. Neither is a fault.
What does the amber light on a Lennox furnace mean?
On an ML296, the amber light counts airflow. Each flash is 100 CFM, so five flashes is 500 CFM.
What does a solid light on a Lennox furnace board mean?
On an ML296, a steady red light is normal operation. On the newer ML180UHE board, a light that stays on means the board found a fault. Check the model sticker first.
