The short answer
To reset a Lennox furnace in Northern Virginia, take a photo of any code first, then move the thermostat from Heat to OFF and back to Heat. That’s how Lennox says to break its one-hour lockout. If that doesn’t do it, turn the furnace’s power switch off, count to ten, and turn it back on. Most Lennox furnaces don’t have a reset button for a lockout at all. The buttons you can find inside are safety switches, and a tripped one means something needs fixing before the furnace runs again.
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).
The reset Lennox describes, step by step
A Lennox furnace that can’t light tries five times, then locks the gas valve out and waits an hour. Lennox calls that hour Watchguard. When it’s up, the furnace starts over on its own if the thermostat still wants heat. You don’t have to wait it out:
- Photo the code. On an SL280, EL296 or SLP99 it’s an E and three digits on the small display behind the lower door. On a Merit furnace (ML180, ML195, ML296) it’s a red light blinking a count. 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.
- Thermostat from Heat to OFF, then back to Heat. Lennox’s own wording for the ML180UH and ML296 is to move the thermostat “from ‘Heat’ to ‘OFF’ then back to ‘Heat’.” The lighting sequence starts again from the top. On the ML296, Lennox says the call for heat has to be gone for more than 3 seconds, so give it a slow count of ten.
- Still nothing? Cut the power. The furnace switch looks like a light switch, on the wall beside the furnace or on the cabinet. Off, count to ten, on. Lennox says removing power from the control for more than 1 second resets an ignition lockout on the ML296.
- Listen. The small inducer fan starts, you hear a click, the igniter glows orange, and the burners light within about a minute. The big blower comes on shortly after. If it lights and keeps running, you’re done for now.
Doesn’t light, or locks out again? Stop there. One reset is a fair test. A second lockout tells you the cause is still there, and the code you photographed tells a tech where to look.
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| What you see | What it is | Should you press it? |
|---|---|---|
| A small button on a Merit control board | The diagnostic button. A short press replays the stored codes. Holding it more than 5 seconds erases them. | Not to fix anything. Erasing the codes erases the clues. |
| A button on the face of a small breaker in the control box (SL280, SLP99) | A 3-amp breaker on the 24-volt control circuit. When it trips, the whole furnace shuts down. | Once. If it trips again, a wire is shorting somewhere and a tech needs to find it. |
| A button on a switch mounted on the burner box | A flame rollout switch. On an SL280 or SLP99 it trips at 210°F. Lennox ties a trip to a blocked heat exchanger, flue or air supply. | No. Call. (E200 rollout lockout) |
| A small switch the blower door presses in | The door switch. With the blower door off, the furnace shuts down on purpose. | No need. Just make sure the door is on and seated. |
The limit switches that shut the burners off when the furnace runs too hot reset themselves, so there’s no button for those. If you’re seeing E250, start with the filter.
One more on SL280 and SLP99 furnaces: a variable-speed blower motor tries to start eight times. After the eighth, its controller locks out, and Lennox says to reset it by turning the power off for a moment. If it locks out again, the motor or the wheel is the problem (E292 blower motor).
When a reset is the wrong move
- Any gas smell. Out of the house first, then call from outside.
- E200, or the rollout blink count on a Merit (8 flashes on an ML180UHE, 5 on an ML180UH or ML195, 4 on an ML296, where it shares the count with the limit). That’s a hard lockout for a reason.
- E118 on an SLP99. Lennox turns the furnace into a hard lockout when someone resets it six times in 15 minutes. The furnace is telling you resets aren’t working.
- Water on the floor around the furnace. A clogged condensate drain trips a safety on purpose. A reset won’t clear it; the drain will.
- A carbon monoxide alarm. Everyone out, then call.
What keeps tripping it, and what it costs
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| What’s behind the lockout | Codes you’ll see | Our price |
|---|---|---|
| Cracked or worn-out igniter | E207, E290, or a no-ignition blink count | $240 to $410 |
| Dirty flame sensor | E240, E270, E273 | $95 to $180 |
| Clogged condensate drain | E223, E227, E271 | $175 to $420 |
| Clogged filter or blocked airflow | E250, E274 | Often just a filter |
| Control board failed | E125 that comes back after a power cycle | $550 to $1,180 |
| Blower motor won’t start | E292 | $670 to $2,400 |
The diagnostic is a flat $99, and you see the repair price in writing before we start. Most lockouts turn out to be the igniter, the flame sensor or the drain, and those parts are usually on the van. Lennox furnace not igniting walks through the lighting sequence step by step.
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 where a weak igniter or a slow drain gets caught before a 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
Most Lennox furnaces don’t have a reset button for a lockout. You reset it at the thermostat (Heat to OFF and back to Heat) or at the furnace power switch. The buttons inside are a diagnostic button on Merit boards, a 3-amp control breaker on SL280 and SLP99 furnaces, and the rollout switches, which shouldn’t be reset until someone finds why they tripped.
How long do I turn off a Lennox furnace to reset it?
Lennox says more than 1 second of power off resets an ignition lockout on an ML296, or more than 3 seconds with the thermostat’s heat call removed. A slow count of ten covers both.
Will a Lennox furnace reset itself?
Yes, after an ignition lockout. Lennox’s Watchguard waits 60 minutes and tries again if the thermostat still wants heat. A hard lockout, like E200, stays off until someone turns the power off and back on.
Why does my Lennox furnace keep locking out after a reset?
Because the cause is still there. Two lockouts in a row usually means a cracked igniter, a dirty flame sensor, a blocked vent or drain, or a gas supply problem. Photo the code and call.
