The short answer
E240 on a Lennox furnace in Northern Virginia means low flame current in run mode. The burners are lit, but the flame sensor is barely reading them. Most of the time the sensor rod has a coating on it and needs cleaning, a few minutes of a tech’s time. Left alone, it tends to turn into a furnace that lights and drops out, and then into an E270 lockout on a cold night.
How a flame sensor works
The flame sensor is a thin metal rod that sits in the flame at the end of the burners. The control board sends a tiny current through it, and the flame carries that current back. It’s measured in millionths of an amp. If the board can’t read enough of it, it can’t prove there’s a flame, so it shuts the gas off to be safe. That’s the right call when there really is no flame, and an annoying one when the rod is just dirty.
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What Lennox says E240 and E241 mean
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| Code | What Lennox says | What it means for you |
|---|---|---|
| E240 | Low flame current, run mode | The sensor reads a weak flame. It clears after the current heat call ends, and it’ll come back until someone fixes it. |
| E241 | Flame sensed out of sequence, flame still present | The opposite problem: flame where there shouldn’t be one. Lennox’s fix starts with shutting off the gas and checking the gas valve. Turn the gas to the furnace off and call. |
On a Merit furnace with a red light, an ML180UHE blinks 4 flashes for a low flame signal and 12 flashes for flame sensed without the gas valve on. On an ML180UH, ML195UH or ML296UHV, 1 flash means flame with the gas valve off. Treat any of those last ones like E241.
Why the flame current drops
A coated sensor. The most common one. A thin film builds up on the rod over a season or two, and it insulates the rod from the flame.
A poor ground. The flame current has to get back to the board through the furnace’s ground. Lennox’s own fix list for E240 includes checking the neutral-to-ground voltage and the unit ground. A loose ground wire gives you a low reading with a clean sensor.
A weak or lazy flame. Low gas pressure or dirty burners make a flame that doesn’t wrap the rod the way it should.
A worn sensor or wire. A cracked porcelain base or a corroded wire end. The sensor gets replaced.
Can you clean it yourself?
Handy homeowners do. If you try it, turn the furnace switch off first, take out the one screw that holds the rod, and rub the metal with a fine abrasive pad until it’s bright. Don’t use coarse sandpaper, don’t bend the rod, and don’t touch the igniter next to it. If E240 comes back within weeks, cleaning wasn’t the whole answer, and the ground or the gas pressure needs a meter.
What it usually costs
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| The fix | Our price |
|---|---|
| Flame sensor cleaned | $95 to $180 |
| Igniter or flame sensor replaced | $240 to $410 |
| Control board replaced | $550 to $1,180 |
The diagnostic is a flat $99. When we clean a sensor we read the microamps before and after, so you know it’s fixed and not just better for a week. More on this part in flame sensor problems.
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.
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
What does E240 mean on a Lennox furnace?
Low flame current in run mode. The burners are lit, but the flame sensor isn’t reading enough current. A dirty sensor is the usual cause; a poor ground or low gas pressure are the others.
Can I clean my Lennox flame sensor myself?
Yes, if you’re handy. Turn the power off at the furnace switch, remove the rod, and polish it with a fine abrasive pad. Don’t bend it or touch the igniter. If the code comes back quickly, have the ground and gas pressure checked.
What does E241 mean on a Lennox furnace?
Flame sensed out of sequence, with flame still present. The furnace sees a flame when the gas valve should be closed. Turn the gas to the furnace off and call a technician.
Why does my Lennox furnace light and then shut off?
If the burners light and go out within a few seconds, the control can’t prove the flame, almost always because the flame sensor is dirty or the ground is poor.
