The short answer
When a Lennox furnace in Northern Virginia won’t ignite, a cracked igniter is the most common reason, followed by a dirty flame sensor and a gas or venting problem. The furnace tries to light five times. If none of them catch, it locks out and tries again in an hour. The code on the board, or the blink count on a Merit, tells you which step of the lighting sequence failed, and that points straight at the part.
If you smell gas, skip all of it. Leave the house and call the gas company and 911 from outside. If a carbon monoxide alarm is going off, get everyone out first (what to do when the CO alarm sounds).
How a Lennox furnace lights
Lennox prints the same sequence in its ML180UH, ML296 and SL280 manuals:
- The thermostat calls for heat and the small inducer fan starts.
- A pressure switch proves the inducer is pulling air through the vent.
- After a 15-second purge, the igniter heats up.
- After 20 seconds of warm-up, the gas valve opens for a 4-second try.
- The flame sensor proves there’s a flame, and the big blower comes on a little later.
Miss at step 4 or 5 and the control tries again. On SL280 and SLP99 furnaces there’s 35 seconds between tries. After five tries in all, it locks the gas valve out for an hour; Lennox calls that Watchguard.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
Where it stops tells you what’s wrong
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| What you see and hear | Where it stopped | Lennox code |
|---|---|---|
| Inducer runs, then nothing else | The pressure switch never closed: vent, drain or inducer | E223, E227, E271 (pressure switch codes) |
| Inducer runs, igniter never glows | The igniter, or the board circuit that powers it | E207, E290 |
| Igniter glows, gas clicks on, nothing lights | Gas supply or the gas valve | E270 |
| Burners light for a few seconds and go out | The flame sensor can’t see the flame | E240, E270, E273 (E240 low flame current) |
| No inducer, no sound at all | Thermostat, power, the door switch or the control | E370, or none |
A Merit furnace has no E-codes. Its red light blinks a count instead, and the count changes by model: no ignition is 7 flashes on an ML180UH, ML195UH or ML296UHV, and 3 flashes on an ML180UHE. Lennox flashing red light has the full tables.
What to check before you call
Is the gas on? If the stove lights or the water heater makes hot water, gas is reaching the house. The valve on the pipe into the furnace should have its handle in line with the pipe. Sideways means it’s shut. If you don’t know who shut it or why, leave it off and call us.
Is the thermostat calling? Heat mode, set a few degrees warmer than the room.
Is the furnace powered? Furnace switch on, breaker on, blower door on and seated.
Any water near the furnace? On a high-efficiency furnace, a clogged condensate trap keeps the pressure switch from closing, and the furnace won’t even try to light. Furnace condensate drain clogs shows what that looks like.
Photo the code, then reset once. Thermostat from Heat to OFF and back to Heat. Take a photo of the code before you reset anything. A reset can clear what’s showing, and that number saves the tech time and you money. How to reset a Lennox furnace has the details.
Don’t touch the igniter. Older ones crack if you bump them, and the oil from a finger shortens the life of a new one. And don’t keep resetting it; a furnace that didn’t light on the fifth try won’t light on the fifteenth.
What it usually turns out to be
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| The cause | How a tech confirms it | Our price |
|---|---|---|
| Cracked or worn-out igniter | Measures its resistance. Open or out of range, it’s replaced. | $240 to $410 |
| Dirty flame sensor | Burners light and drop out. Flame current reads low. | $95 to $180 |
| Clogged condensate trap or drain | Water backed up, pressure switch open | $175 to $420 |
| Control board not powering the igniter | Voltage at the igniter plug during a try | $550 to $1,180 |
| Blocked vent or weak inducer | Pressure at the switch, vent checked end to end | Priced in writing |
| Gas valve or gas pressure | Pressure measured at the valve | Priced in writing |
Most no-light calls are the igniter or the flame sensor, and both are usually on the van. Flame sensor problems goes deeper on the second one. 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. A yearly check is when a weak igniter gets caught before the first cold night.
If the repair is a big one on an older furnace, get a second look before you spend it. Our free second opinion costs you nothing, and the estimator shows what a new system costs with $1,500 off already in the price. Thinking about brands? Lennox vs Trane lays out the difference.
Every Lennox code in one place: Lennox error codes. Other brands too: furnace and AC error codes.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
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 won’t my Lennox furnace ignite?
A cracked igniter is the most common reason, then a dirty flame sensor, then a gas or venting problem. The code on the board says which step of the lighting sequence failed.
How many times does a Lennox furnace try to light?
Five tries in all on Lennox’s ML180UH, ML296, SL280 and SLP99 furnaces. Then it locks out for 60 minutes and starts over if the thermostat still wants heat.
My Lennox furnace lights, then shuts off. Why?
Almost always the flame sensor. When it’s coated, it can’t prove the flame, so the furnace shuts the gas off within seconds. Cleaning it runs $95 to $180.
How much does a Lennox igniter replacement cost?
An igniter replacement runs $240 to $410. The diagnostic is a flat $99, and you see the price in writing before any work starts.
