Next Phase Heating is an independent HVAC contractor. We are not an authorized dealer for, or affiliated with, any equipment manufacturer. Brand names are used only to identify the equipment we service. Codes vary by model and model year — the legend on your own furnace is always the last word.
First — which of the four does your Lennox use?
Lennox has run four genuinely different diagnostic systems across its range. They do not translate into each other. Getting this right is the whole job.
A display showing E followed by three numbers
The Signature and Elite lines, and the newest Merit models. There is a push button next to the display: hold it to reach the menu, and the E option recalls the last ten codes. A decimal point blinking once a second is idle and completely normal.
One red light, one to nine flashes
The Merit line. This is the system with three incompatible versions, described below. There is a button on the board that replays the last five faults.
Two lights, described as DS1 and DS2
The older SureLight boards. Codes are combinations — one light slow, the other steady, and so on. Lennox itself warns in capital letters to identify which light is which against the board layout first, because a slow flash on the left and a slow flash on the right mean different things.
One light, flashing twice in a pattern
Some SureLight boards flash a number, pause, flash a second number, then a longer pause. Five then one is a rollout lockout on these. We have only found this table in a single Lennox document, so we would want to see your unit before relying on it.
What the codes mean
Stop, and call someone
These are the codes where the furnace has found flame somewhere it should not be. Lennox’s own remedy for one of them begins with the words shut off gas.
E200
Hard lockout — the rollout circuit is open, or has been open
On the display models this is unambiguous and it is the serious one. The furnace has locked itself out because flame reached where it must not. Leave it off.
E241 · E275
Flame sensed out of sequence, and still present
Lennox’s published remedy for E241 starts with shutting off the gas and checking the valve for a leak. That is the manufacturer’s own instruction, not our caution. E275 is the lockout that follows it.
3, 4 or 5 flashes — depending on your model
On the single-light Merit boards, rollout does not sit at a fixed number
This is the single most important thing on this page. On the ML193UH, five flashes is the rollout switch. On the ML296UHV as documented in 2020 and 2021, three flashes covers rollout or a limit. On the same ML296UHV model in the 2024 manual, rollout moved to four flashes and five is no longer used at all. So a Lennox flashing three, four or five deserves the careful answer until you have matched it to your own furnace.
E204 · E205
The gas valve is miswired, or its control relay has shorted closed
Both are gas valve faults on the control side. Not something to reset past.
Usually you, and usually the filter
Lennox is unusually good here — on the display models it has codes that say restricted airflow in as many words, and even a warning code that fires before anything shuts down.
E311
The furnace has quietly turned its own heat output down to match the air it can move
This is a warning rather than a fault, and the furnace keeps running. Lennox’s remedy is to replace the filter or repair the restriction. It is the earliest warning you will get that a filter needs doing, and almost nobody ever sees it because nobody checks the display when the furnace is working.
E291 · E312
Restricted airflow — the blower cannot move what it needs to
E291 is in heating, E312 in cooling or continuous fan. Filter, then every supply register open, then check nothing is against the return.
E250
The limit switch circuit is open
On the display models this is the limit on its own, with rollout reported separately as E200 — which is a genuine advantage of these boards over most other brands. Treat it as an airflow problem: filter, registers, return.
E370
An interlock switch has been open for two minutes
This is usually the blower door. If someone has had the panel off and not clicked it fully back on, this is the code you get.
E223 · E227
The low pressure switch failed to close, or opened during the trial for ignition
Go outside and look at the white intake and exhaust pipes for nests, snow or debris.
Needs a technician, but it is not an emergency
The display models carry a long list of component codes. These are the ones that come up most in a house.
7 or 8 flashes, single-light models
Lockout — it failed to light, or lost the flame too many times
The furnace has given up rather than kept trying. One reset to let it retry is fair. Two means something needs finding.
6 flashes, single-light models
Pressure switch cycle lockout
It has opened and closed too many times in one cycle. Often venting, sometimes the switch.
9 flashes, single-light models
Line voltage polarity is incorrect
A wiring problem rather than a furnace problem.
1 flash, single-light models
Flame present with the gas valve de-energised
Note that on the Merit single-light boards this serious fault is at one flash, which is the opposite end of the scale from where most people expect to find it.
E105 · E110 to E126
Communication and power supply faults on the connected models
These appear on the Signature and Elite units that talk to a communicating thermostat. They are usually wiring or a board, not the burner.
E150 · E151
Refrigerant leak detection on the newest models
The 2024 and later units carry sensors for the new refrigerants. This is the system reporting a sensor or a detection event, and it is a call rather than a wait.
Five ways a Lennox code gets read wrong
- The same model changed its own chart between manual revisions. The ML296UHV installation instructions dated 2020 and 2021 put rollout or limit at three flashes. The 2024 revision for the same model name puts primary, secondary or rollout at four flashes and drops five entirely. Searching by model name alone is not enough on this brand.
- A blinking decimal point is the furnace at rest. On the seven-segment models, a decimal point flashing once a second means idle. It is the most-reported non-fault on Lennox equipment.
- The amber light is counting airflow, not faults. On the Merit boards there is an amber light that flashes once per hundred CFM. Someone counting nine flashes there is reading nine hundred CFM, not code nine.
- On the two-light boards, which light is which decides the answer. Slow flash on the first with the second steady, versus steady on the first with the second flashing slowly, are two different faults — and on some board generations they are limit and rollout respectively. Lennox prints its own warning about this. If you cannot tell DS1 from DS2 on the board, you do not have a reliable code.
- Opening the blower door can create a code all by itself. The interlock fires on the display models as E370. Perfectly harmless, but confusing if it appears while you are investigating something else.
There is no error code for a cracked heat exchanger
It is the most searched question about furnaces and one of the most misused in a sales conversation, so plainly: no Lennox control board detects a cracked heat exchanger. Neither does any other manufacturer’s board. The code does not exist. What a board reports is that the rollout circuit opened, or that the limit keeps tripping, and both can be downstream signs of an exchanger that has failed. The crack itself is found by inspection, with a camera and a combustion analyser, by someone who has opened the furnace up. If a replacement quote arrives on the strength of a code alone, get a second number before you spend anything.
Three, four or five flashes on a Merit?
That is the one worth having looked at rather than guessed. Book it and tell us the model number off the door.
Book a Furnace Diagnostic60 seconds online · or call (206) 486-6373
How it works when you book
- 1
Read the code before you reset anything The moment you cycle the power the live code is gone. Count it twice and write it down.
- 2
Book online and tell us what it showed Takes about a minute. Put the code in the notes — it changes what the technician loads into the van.
- 3
You get the price before we start Diagnosed, explained, and quoted in writing. Nothing happens until you say go.
Our service areas
We run out of Covington and cover the greater Puget Sound area.
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Covington, Kent, Renton, Maple Valley, Auburn, Federal Way, Des Moines, Enumclaw, Bellevue, Issaquah, Sammamish, Bothell
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Snohomish County
Lynnwood, Everett
Not on the list? We cover the greater Puget Sound area around our Covington base. Book it and a technician confirms within 1 business hour.
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Quick answers
Why does my Lennox chart not match the one I found online?
Because Lennox runs four different diagnostic systems and has revised at least one of them within the same model name. A chart written for a Signature model with a display tells you nothing useful about a Merit with a single blinking light, and a chart for a 2021 Merit can put a safety fault at a different flash count than the 2024 version of the same furnace. Match the model number on the door, not the brand.
I have the Lennox extended warranty. Does that change anything?
Yes, and we would rather tell you now than after we have been out. Lennox’s ordinary warranty is open — their own guidance says homeowners can work with any licensed installer. But the separate extended coverage they sell is different: their terms say the owner must use a participating Lennox dealer, and that Lennox has no liability under it if you do not. We are not a Lennox dealer. So if you bought that extended plan, the repair should go through a dealer to be covered, and it is worth checking your paperwork before booking anyone. If you only have the standard warranty, none of this applies and we are happy to help.
Where do I find which model I actually have?
On the rating plate inside the furnace door, and usually on a sticker on the outside of the cabinet too. Write the whole string down — the difference between ML193UH and ML296UHV is exactly the difference this page is about.
How do I get the stored codes out of a Merit board?
There is a push button on the control. Press and release it and it flashes the last five faults, most recent first. Holding it for more than five seconds clears the history, so be careful not to hold it while you are trying to read it.
Can you work on a Lennox at all if you are not a Lennox dealer?
Yes. Lennox’s own consumer guidance says homeowners have the option to work with any licensed professional installer, and their standard warranty is written that way. The only carve-out is the extended plan described above.
Tell us the model number as well as the code.
Book online in about a minute. On a Lennox the model number is what makes the code mean anything, so put both in the notes.
Book a Furnace Diagnostic60 seconds online · or call (206) 486-6373
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