Lennox furnaces either flash a red light on the control board (a burst of quick flashes, 3 seconds off, repeat) or show an E code such as E250 on a seven-segment display or a communicating thermostat. Count the flashes or note the code, check power, gas and the filter, then restart once by moving the thermostat from Heat to Off and back. If it returns, call a technician.
Lennox furnaces report faults in one of two ways. Models with a single red light on the control board flash a code: the light blinks a set number of times, goes dark for 3 seconds, then repeats. Models with a seven-segment display, such as the SL280UHV, show an E code like E250, and on communicating systems the same number appears on the Lennox thermostat. Count or note the code, then run the manual's failure-to-operate checks below before calling a technician.
The flash codes here come from Lennox's installation instructions for the ML180UHE (April 2024) and the older ML180UH (July 2018). The E codes come from the SL280UHV installation instructions and Lennox's alert code guide for communicating systems.
If you smell gas, stop here
Lennox's ML180UHE installation instructions say that if you smell gas, stop and immediately call your gas supplier from a neighbor's phone, then follow the supplier's instructions. Leave the house first and make the call from outside. Carrier's homeowner guide to a furnace that won't turn on gives the same advice: evacuate and call from a safe place if you smell rotten eggs or a carbon monoxide alarm sounds.
How to read the Lennox flash code
On the ML180UHE controls, a slow "heartbeat" (1 second on, 1 second off) means the furnace is idle, running the fan or cooling. A fast heartbeat (half a second on, half off) is a normal call for heat. An error is a burst of quick flashes at half-second intervals, then 3 seconds off, then the same burst again. Count the flashes in one burst.
The light sits on the integrated control board next to a small recall button. Lennox says the board stores the last 10 error codes, even through a power cut. Pressing and releasing the button shows the most recent code first, then the older ones with a 3-second gap between them. Holding it for more than 5 seconds clears the history, so press briefly. The board sits inside the cabinet, so a technician usually does the recall.
Lennox flash codes (ML180UHE)
These are from the diagnostic code tables for the two ignition controls in the ML180UHE manual, parts 103217-03 and 107163-01, which give the same meanings. The "what you can check" column only holds steps from the manual's own failure-to-operate list.
| Code | What it means | What you can check | Call a pro? |
|---|---|---|---|
| Off | No power to the control, or a board fault | Disconnect switch on, breaker or fuse OK, access panels in place | Yes, if power is on |
| On, steady (107163-01) | Board fault detected | Nothing | Yes |
| Slow heartbeat | Normal: idle, continuous fan or cooling | Nothing | No |
| Fast heartbeat or rapid flash | Normal: call for heat, burners running | Nothing | No |
| 1 flash | Line voltage polarity reversed | Nothing | Yes |
| 2 flashes | Improper earth ground | Nothing | Yes |
| 3 flashes | Burner failed to light, or lost flame during a heat call | Gas on at the meter; manual shut-off valves open | Yes, if it returns |
| 4 flashes | Low flame signal; check flame sensor | Nothing | Yes |
| 5 flashes | Watchguard: burner failed to light, too many retries or recycles; 1-hour lockout | Thermostat Heat, Off, then Heat to restart once | Yes, if it returns |
| 6 flashes | Not used | Nothing | No |
| 7 flashes | Primary or secondary limit open, or limit open longer than 3 minutes (Watchguard) | Replace a dirty or plugged filter | Yes, if it returns |
| 8 flashes | Rollout switch open | Nothing | Yes |
| 9 flashes | Pressure switch failed to close, or opened during a heat call | Flue outlet clear of blockages | Yes |
| 10 flashes | Watchguard: pressure switch opened 5 times in one heat call | Flue outlet clear of blockages | Yes |
| 11 flashes | Pressure switch stuck closed before the inducer started | Nothing | Yes |
| 12 flashes | Flame sensed with the gas valve off | Nothing | Yes |
| 13 flashes | Low line voltage | Nothing | Yes |
Everything marked "Yes" goes to a licensed technician. The manual's own list says to call the service technician if the unit locks out again or if the flame rollout switches are tripped.
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Older Lennox boards count differently
Don't use the table above on an older furnace without checking. Lennox's ML180UH installation instructions from 2018 show a different control with different numbers. On that board, 4 flashes means an open limit, not a low flame signal.
| Code (ML180UH) | What it means | What you can check | Call a pro? |
|---|---|---|---|
| LED off | No power to the control, or a control hardware fault | Power switch, breaker, panels in place | Yes, if power is on |
| 1 flash | Flame present with the gas valve off | Nothing | Yes |
| 2 flashes | Pressure switch closed with the inducer off | Nothing | Yes |
| 3 flashes | Pressure switch open with the inducer on | Nothing | Yes |
| 4 flashes | Primary limit switch open | Replace a dirty filter | Yes, if it returns |
| 5 flashes | Rollout switch open | Nothing | Yes |
| 6 flashes | Pressure switch cycle lockout | Nothing | Yes |
| 7 flashes | Lockout, burners fail to light | Gas on at the meter and valves open | Yes, if it returns |
| 8 flashes | Lockout, burners lost flame too many times | Nothing | Yes |
| 9 flashes | Line voltage polarity incorrect | Nothing | Yes |
That board stores the last 5 codes. The 2018 manual's table lists "LED off" twice, once for no power and once for normal operation, so check your own model's instructions for how its light looks when everything is working.
Lennox E codes (SureLight display and iComfort systems)
Furnaces like the SL280UHV use a SureLight control with a seven-segment display. Its installation instructions say the display shows status, airflow and error codes, and that the furnace pairs with the Lennox iComfort Touch thermostat in communicating setups. These are the heating and power codes from its diagnostic table:
| Code | What it means | What you can check | Call a pro? |
|---|---|---|---|
| E110, E113 | Line voltage low (E110) or high (E113) | Nothing | Yes |
| E111 | Line voltage polarity reversed | Nothing | Yes |
| E112, E117 | Ground not detected (E112, system shuts down) or poor ground (E117, warning) | Nothing | Yes |
| E115, E116 | 24-volt power low (E115) or high (E116) | Nothing | Yes |
| E125, E126 | Control failed self-check, or internal communication problem | Cycle power once | Yes, if it repeats |
| E200 | Hard lockout: rollout circuit open or previously open | Nothing | Yes |
| E201, E202 | Can't communicate with the blower motor (E201); motor doesn't match unit size (E202) | Nothing | Yes |
| E203 | Unit capacity not programmed | Nothing | Yes |
| E204 to E206 | Gas valve miswired (E204), relay contact shorted (E205), second-stage relay failure (E206) | Nothing | Yes |
| E207, E290 | Hot surface igniter open (E207); igniter circuit fault (E290) | Nothing | Yes |
| E223 to E227 | Pressure switch faults: low switch open (E223) or stuck closed (E224), high switch open (E225) or stuck closed (E226), low switch open during ignition or run (E227) | Nothing | Yes |
| E228 | Combustion air inducer calibration failure | Nothing | Yes |
| E229 | Ignition on high fire; no action needed | Nothing | No |
| E240 | Low flame current in run mode | Nothing | Yes |
| E241, E275 | Flame sensed out of sequence, still present (E241) or now gone (E275) | Nothing | Yes |
| E250 | Limit switch circuit open | Replace a dirty filter | Yes, if it returns |
| E252 | Discharge air temperature too high | Replace a dirty filter | Yes, if it returns |
| E270 | Soft lockout: too many retries, no flame sensed. Watchguard resets after one hour | Gas on at the meter; shut-off valves open | Yes, if it returns |
| E271 to E274 | Soft lockout after too many retries or recycles: pressure switch (E271, E272), flame failure (E273), limit open (E274) | Replace a dirty filter (E274) | Yes |
| E276 | Watchguard calibration failure | Nothing | Yes |
| E291 | Heating airflow restricted below the minimum | Replace a dirty filter | Yes, if it returns |
| E292, E295 | Blower motor can't start (E292) or is too hot (E295) | Nothing | Yes |
| E294 | Combustion air inducer over current | Nothing | Yes |
| E311 | Heat rate reduced to match low airflow | Replace a dirty filter | Yes, if it returns |
| E370 | Interlock switch (such as a float switch) open for 2 minutes | Nothing | Yes |
The same table also lists communication codes (E105, E120, E124, E131), sensor and cooling codes (E180, E310, E312, E313) and outdoor relay codes (E334 to E349). Those point to wiring or setup and belong to the technician.
Lennox's alert code guide for communicating systems (March 2023) uses the same numbers on its smart thermostats. It sorts them by priority: a "Service Urgent" alert, such as 200 or 270, shows on the homeowner's thermostat and means a dealer visit is needed to get the system running. "Service Soon" alerts, such as 223 or 250, usually recover on their own and show only on the dealer screen.
What to do for the common codes
5 flashes or E270: Watchguard lockout
The burner failed to light after repeated tries, so the control waits 60 minutes and then tries again on its own. Lennox says this lockout is usually caused by low gas line pressure. To restart it sooner, the manual says to move the thermostat from Heat to Off and back to Heat. Before you do, check that gas is on at the meter and the manual shut-off valves are open. On SL280UHV display models the manual says the Watchguard lockout resets itself after one hour. If it locks out again, Lennox says to call the service technician to inspect for blockages.
7 flashes or E250: limit open
The furnace overheated and the limit switch cut the burners. The manual's checklist says dirty or plugged filters will cause the limit control to shut the unit off, and the alert guide lists a dirty filter or blocked ductwork for code 250. Turn the power off and replace the filter. Lennox's maintenance list calls for checking filters monthly. Our guide on how often to change a furnace filter covers sizes. If the code returns on a clean filter, call a technician.
9 flashes or E223 and E227: pressure switch
Lennox's checklist notes that an obstructed flue will shut the unit off at the pressure switch, and tells owners to check the flue and outlet for blockages. The rest of the alert guide's list, vent pressure, the inducer and the condensate drain, is technician work.
8 flashes or E200: rollout switch
Lennox's checklist says to call the service technician if the flame rollout switches are tripped. The alert guide's steps for code 200 start with correcting the cause of the trip and checking for a blocked vent, so don't reset it yourself.
Models this covers
- Lennox ML180UHE (controls 103217-03 and 107163-01): the main flash code table.
- Lennox ML180UH (2018 instructions): the older flash code table.
- Lennox SL280UHV with SureLight display: the E code table.
- Lennox communicating systems: alert code numbers from the alert code guide.
Lennox changes boards between series and years, so the chart in your own model's installation instructions wins over any table here. If the furnace shows no code and won't start, work through our furnace won't turn on checklist. If it runs but blows cold air, see why a furnace blows cold air. We also list Carrier furnace codes, Goodman furnace error codes and Trane furnace error codes.
Frequently asked questions
How do I reset a Lennox furnace error code?
Lennox's ML180UHE manual says to move the thermostat from Heat to Off and back to Heat to interrupt the 60-minute Watchguard lockout. The control also restarts on its own after an hour. Before that, check that gas is on at the meter, the shut-off valves are open and the filter is clean. If the furnace locks out again, Lennox says to call a service technician.
What does E250 mean on a Lennox furnace?
E250 means the limit switch circuit is open, usually because the furnace is overheating. Lennox's alert guide lists low supply air from a dirty filter or blocked ductwork, high gas pressure and the wrong firing rate. Replace a dirty filter first. If the limit stays open longer than 3 minutes the furnace goes into a one-hour soft lockout, and a repeat needs a technician.
What does E270 mean on a Lennox furnace?
E270 is a soft lockout: the furnace exceeded its maximum number of ignition retries without sensing a flame, so it is in Watchguard mode. Lennox's alert guide says to check gas flow, the igniter, the flame sensor current and the filters. Check that gas is on at the meter and the valves are open. If it keeps happening, call a technician.
What does a slow blinking red light mean on a Lennox furnace?
On the ML180UHE controls, a slow heartbeat of 1 second on and 1 second off is normal. It means the furnace is idle, running the fan only or cooling. A faster heartbeat or steady rapid flashing during a call for heat is also normal. An error looks different: a burst of quick flashes, 3 seconds dark, then the same burst again.
Why do Lennox flash codes differ between furnaces?
Lennox has used different control boards over the years. On the ML180UHE controls, 4 flashes means a low flame signal, while on the board in the 2018 ML180UH manual, 4 flashes means an open primary limit. Always match the code to the chart for your own model, which is printed in its installation instructions.
Is it safe to keep running a Lennox furnace with an error code?
Lockout and limit codes stop the burners, so the furnace isn't heating anyway. Restart it once after checking power, gas and the filter. If the code comes back, leave the furnace off and call a technician, especially for a rollout code. If you smell gas, leave the house and call your gas supplier from outside or a neighbor's phone.
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