Lennox Furnace Error Codes

Updated October 13, 2026 · by Aurora Star Tech Inc.

170 Lennox Furnace error codes and diagnostic codes, with causes and troubleshooting steps for each. Select a code, or find your model below.

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Lennox Furnace error codes list

Code	Meaning	Severity
--	Double dashes - zone sensor problem	Medium
1 flash	Line voltage polarity reversed	Medium
2 flashes	Improper earth ground	Medium
3 flashes	Burner failed to light or lost flame	High
4 flashes	Low flame signal	Medium
5 flashes	Watchguard - ignition retries/recycles exceeded	High
7 flashes	Primary or secondary limit open	High
8 flashes	Rollout switch open	Critical
9 flashes	Pressure switch stays open or drops out on a heat call	High
10	Unknown device detected	High
10 flashes	Watchguard - pressure switch opened 5 times in one heat demand	High
11	Missing device	High
11 flashes	Pressure switch stuck closed before inducer starts	Medium
12	Indoor unit not detected	High
12 flashes	Flame sensed with gas valve off	Critical
13	Duplicate comfort sensor ID	High
13 flashes	Low line voltage	Medium
14	Too many devices of the same type	High
15	Parameter mismatch detected	Low
20	Protocol upgrade required	High
21	Incompatible equipment detected	High
29	Over temperature protection (indoor above 90 F)	High
30	Low temperature protection (below 40 F)	High
31	Lost communication with device	High
32	Device resetting	Low
34	Must program unit capacity for device	High
35	Incorrect operation of device	High
36	Heating when not requested	High
37	Cooling when not requested	High
38	Not heating when requested	High
39	No cooling when requested	High
40	HP heating lockout	Low
41	Device control board replaced	Low
500	PureAir differential pressure sensor fault	Medium
501	PureAir UV sensor fault	Medium
502	PureAir UV lamp off	Medium
503	PureAir filter life at 10%	Medium
504	PureAir filter life at 0%	Medium
505	PureAir model selection changed	Medium
506	PureAir lamp at 0% life	Medium
507	PureAir filter calibration failure	Medium
530	Zoning low damper 24 VAC	Medium
532	Zoning pressure switch opened (high pressure)	Low
542	Zone 1 temperature sensor fault	Medium
543	Zone 2 temperature sensor fault	Medium
544	Zone 3 temperature sensor fault	Medium
545	Zone 4 temperature sensor fault	Medium
546	Zone sensor parameters reset after power restore	Medium
547	Zone sensor parameters reset from system interruption	Medium
548	Zone humidity sensor error	Medium
551	Zone sensor lost communication	Medium
610	Low room temperature detected	High
700	Thermostat temperature sensor problem	High
701	Thermostat temperature above limit	High
703	Thermostat humidity sensor problem	Medium
3000	Replace filter 1	Low
3001	Replace filter 2	Low
3002	Replace humidifier pad	Low
3003	Replace UV bulb	Low
3004	Maintenance reminder	Low
3005	PureAir maintenance	Low
65537	Missing base (smart hub)	High
65538	Missing base	Low
65539	Thermostat lost connection or internal fault	High
65540	Missing HD wall display	Low
65541	Firmware download failed	Low
65542	Firmware update failed	Low
65543	Firmware updated	Low
65544	More than 8 thermostats in a group	Low
70001	IAQ internal sensor fault	Medium
70002	IAQ internal data fault	Medium
70003	IAQ side switch in OFF position	High
70004	Replace Smart IAQ monitor	Medium
70501	Smart room sensor internal data fault	Medium
70502	Smart room sensor internal fault	Medium
70503	Smart room sensor low battery	Low
70504	Smart room sensor battery 1%	Low
71001	Wireless extender internal data fault	Medium
71511	Duct or filter blockage	Medium
71517	Indoor fan motor failure	High
80001	IAQ persistent poor air quality	Low
80002	Unresponsive wireless device fault	Low
80003	Lost communication with wireless device	Medium
80005	Unknown wireless device found	Medium
E105	Device communication problem	High
E106	Twin communication fault	Medium
E110	Low line voltage	High
E111	Line voltage polarity reversed	High
E112	Ground not detected	High
E113	High line voltage	High
E114	Line frequency out of range	High
E115	Low 24 VAC	High
E116	High 24 VAC	High
E117	Poor ground detected	Medium
E120	Unresponsive device	Medium
E124	Lost communication with thermostat	High
E125	Control hardware failure	Critical
E126	Control internal communication problem	High
E130	Configuration jumper missing	High
E131	Corrupted control parameters	High
E132	Device control software fault	High
E150	A2L refrigerant leak alarm	Critical
E151	A2L sensor fault	High
E152	Refrigerant leak sensor #2 fault	High
E154	Refrigerant leak sensor #1 communication lost / wrong DIP setting	High
E155	Refrigerant leak sensor #2 communication lost	High
E160	Refrigerant leak sensor #1 wrong type	Medium
E161	Refrigerant leak sensor #2 wrong type	Medium
E163	Furnace control board failure	High
E164	Low GWP test mode active	Low
E180	Outdoor air temperature sensor failure	Medium
E200	Flame rollout - hard lockout	Critical
E201	Indoor blower communication failure	High
E202	Blower motor mismatch	High
E203	Unit capacity not programmed	Critical
E204	Gas valve miswired	High
E205	Gas valve relay contact shorted	Critical
E206	Gas valve second-stage relay failure	Medium
E207	Hot surface ignitor open	High
E223	Low pressure switch failed open	High
E224	Low pressure switch failed closed	High
E225	High pressure switch failed open	High
E226	High pressure switch failed closed	High
E227	Low pressure switch opened during ignition or run	High
E228	Inducer calibration failure	Medium
E229	Ignition on high fire	Low
E240	Low flame current (run mode)	Medium
E241	Flame sensed out of sequence - flame present	Critical
E250	Limit switch circuit open	High
E252	Discharge air temperature too high	Medium
E270	Soft lockout - no flame after max retries	High
E271	Soft lockout - retries exceeded, pressure switch opened	High
E272	Soft lockout - recycles exceeded, pressure switch opened	High
E273	Soft lockout - recycles exceeded, flame failure	High
E274	Soft lockout - limit opened	High
E275	Soft lockout - flame out of sequence, now gone	Critical
E276	Watchguard calibration failure	High
E290	Ignitor circuit fault	High
E291	Heat airflow below minimum	High
E292	Blower motor will not start	High
E294	Inducer over current	High
E295	Blower motor over temperature	High
E310	Discharge air sensor failure	Medium
E311	Heat rate reduced for airflow	Medium
E312	Restricted airflow in cooling/fan	Medium
E313	Indoor/outdoor capacity mismatch	Low
E331	Global network connection problem	Low
E334	Y1 relay stuck on control	High
E344	IFC relay Y1 stuck	High
E345	Relay O failure	High
E347	No 24 V on Y1 output	Critical
E348	No 24 V on Y2 output	Critical
E349	No 24 V between R and O	Critical
E370	Interlock switch open	High
E380	EIM interlock relay fault	High
E381	EIM interlock relay stuck	High
E382	EIM relay W1 fault	High
E390	Low GWP relay stuck	High
E400	LSOM - compressor internal overload	High
E401	LSOM - long run or low pressure	Critical
E402	LSOM - outdoor pressure trip	High
E403	LSOM - compressor short cycling	Medium
E404	LSOM - locked rotor	High
E405	LSOM - compressor open circuit	High
E406	LSOM - open start circuit	High
E407	LSOM - open run circuit	High
E408	LSOM - welded contactor	High
E409	LSOM - low secondary voltage	High
Red LED off	No power to control or board fault	High
Red LED on steady	Board fault detected (107163-01)	High

Find codes by Lennox model

Furnace troubleshooting guides

Blowing cold air All brands · Furnace
No heat All brands · Furnace
Short cycling All brands · Furnace
Blower motor does not run - Lennox SL280UHV ECM checks Lennox · Furnace
Display shows two horizontal bars (soft disable) - Lennox SL297NV Lennox · Furnace
Furnace will not run (no heat) - Lennox EL195UHNE Lennox · Furnace
Furnace will not run (no heat) - Lennox SL297NV Lennox · Furnace
Furnace will not run - Lennox SL280UHV checklist Lennox · Furnace
Indoor blower does not run - Lennox EL195UHNE Lennox · Furnace
Indoor blower does not run - Lennox ML180UHE Lennox · Furnace
Poor ignition or combustion resonance - Lennox EL195UHNE Lennox · Furnace
Poor ignition or loud combustion resonance - Lennox SL280UHNV Lennox · Furnace
Two horizontal lines on display (soft disable) - Lennox communicating Lennox · Furnace

Lennox Furnace specs and setup data

Filter sizes (filter rack kits) Lennox · Furnace · Lennox EL195UHNE
Filter sizes by cabinet width Lennox · Furnace · Lennox ML180UHE
Filter sizes by cabinet width Lennox · Furnace · Lennox SL297NV
Recommended filter size by cabinet width Lennox · Furnace · EL296UHV
Recommended filter size by cabinet width Lennox · Furnace · Lennox SL280UHV

Homeowner help: Furnace Not Turning On: What to Check · Furnace Blinking Light: What the Flash Codes Mean

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.

Lennox Furnace error codes list

Find codes by Lennox model

Lennox EL195UHNE

EL296UHV

Lennox ML180UHE

Lennox SL280UHV

Lennox SL297NV

Furnace troubleshooting guides

Lennox Furnace specs and setup data

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