Lennox Error Codes & Diagnostic Codes

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

Lennox error codes and diagnostic codes: look up a code to see what it means, the likely causes and the steps to fix it.

Lennox error codes by equipment type

Lennox Air Conditioner error codes

Code	Meaning
10	Unknown device detected
11	Missing device
12	Indoor unit not detected
13	Duplicate comfort sensor ID
14	Too many devices of the same type
15	Parameter mismatch detected
20	Protocol upgrade required
21	Incompatible equipment detected
29	Over temperature protection (indoor above 90 F)
30	Low temperature protection (below 40 F)
31	Lost communication with device
32	Device resetting
34	Must program unit capacity for device
35	Incorrect operation of device
36	Heating when not requested
37	Cooling when not requested
38	Not heating when requested
39	No cooling when requested
41	Device control board replaced
611	High room temperature detected
700	Thermostat temperature sensor problem
701	Thermostat temperature above limit
703	Thermostat humidity sensor problem
3000	Replace filter 1
3001	Replace filter 2
3002	Replace humidifier pad
3003	Replace UV bulb
3004	Maintenance reminder
3005	PureAir maintenance
65537	Missing base (smart hub)
65538	Missing base
65539	Thermostat lost connection or internal fault
65540	Missing HD wall display
65541	Firmware download failed
65542	Firmware update failed
65543	Firmware updated
65544	More than 8 thermostats in a group
65545	Cooling capacity alert
70001	IAQ internal sensor fault
70002	IAQ internal data fault
70003	IAQ side switch in OFF position
70004	Replace Smart IAQ monitor
70501	Smart room sensor internal data fault
70502	Smart room sensor internal fault
70503	Smart room sensor low battery
70504	Smart room sensor battery 1%
71001	Wireless extender internal data fault
71504	Suction temperature sensor incorrect reading
71505	Liquid temperature sensor incorrect reading
71506	Suction pressure sensor incorrect reading
71507	Liquid pressure sensor incorrect reading
71508	Outdoor temperature sensor incorrect reading
71510	Indoor TXV failure
71512	High-pressure switch fault
71513	Low-pressure switch fault
71514	System low on refrigerant charge
71516	Outdoor fan motor failure
71518	Compressor failed to start
71520	Blocked liquid line
80001	IAQ persistent poor air quality
80002	Unresponsive wireless device fault
80003	Lost communication with wireless device
80005	Unknown wireless device found
E105	Wiring issue / communication problem
E114	AC line frequency / distortion problem
E115	Low secondary (24 VAC) voltage
E120	Unresponsive device
E124	Thermostat lost communication to smart hub
E125	Control hardware problem
E126	Control internal communication problem
E132	Device control software fault
E180	Outdoor temperature sensor problem
E181	Outdoor suction pressure transducer fault
E182	Outdoor suction temperature sensor fault
E183	Outdoor liquid pressure sensor fault
E184	Outdoor liquid temperature sensor fault
E400	Compressor internal overload tripped
E401	Compressor long run cycle
E402	System pressure trip
E403	Compressor short-cycling
E404	Compressor rotor locked
E405	Compressor open circuit
E406	Compressor open start circuit
E407	Compressor open run circuit
E408	Compressor contactor welded
E409	Outdoor control low 24 VAC
E410	Open low pressure switch
E411	Low pressure switch strikes lockout
E412	Open high pressure switch
E413	High pressure switch strikes lockout
E414	High discharge line temperature
E415	High discharge line temperature strikes lockout
E417	Discharge sensor faulty
E422	Compressor top cap switch open
E423	Inverter CT circuit fault (inverter code 40)
E424	Liquid line sensor faulty
E426	Excessive inverter alarms
E427	Inverter DC peak fault (inverter code 21)
E428	Inverter high main input current (inverter code 22)
E429	Inverter DC link low voltage (inverter code 23)
E430	Inverter compressor startup fail (inverter code 26)
E431	Inverter PFC fault (inverter code 27)
E432	Inverter DC link high voltage (inverter code 28)
E433	Inverter compressor over-current (inverter code 29)
E434	Inverter communication error to main control (inverter code 53)
E435	Inverter EEPROM checksum fault (inverter code 60)
E436	Inverter high heat-sink temperature (inverter code 62)
E437	Inverter heat-sink temperature sensor fault (inverter code 65)
E438	Inverter PFC input over-current (inverter code 73)
E440	Inverter compressor slowdown - high heat-sink temperature (inverter code 13)
E441	Inverter compressor slowdown - high compressor current (inverter code 14)
E442	Compressor top cap switch strikes lockout
E443	Unit code to inverter model mismatch
E446	Low suction pressure fault
E600	Load shed event
E601	Low ambient operational lockout

Lennox Air Handler error codes

Code	Meaning
--	Double dashes - zone sensor problem
10	Unknown device detected
11	Missing device
12	Indoor unit not detected
13	Duplicate comfort sensor ID
14	Too many devices of the same type
15	Parameter mismatch detected
20	Protocol upgrade required
21	Incompatible equipment detected
31	Lost communication with device
32	Device resetting
34	Must program unit capacity for device
35	Incorrect operation of device
36	Heating when not requested
37	Cooling when not requested
38	Not heating when requested
39	No cooling when requested
41	Device control board replaced
500	PureAir differential pressure sensor fault
501	PureAir UV sensor fault
502	PureAir UV lamp off
503	PureAir filter life at 10%
504	PureAir filter life at 0%
505	PureAir model selection changed
506	PureAir lamp at 0% life
507	PureAir filter calibration failure
530	Zoning low damper 24 VAC
532	Zoning pressure switch opened (high pressure)
542	Zone 1 temperature sensor fault
543	Zone 2 temperature sensor fault
544	Zone 3 temperature sensor fault
545	Zone 4 temperature sensor fault
546	Zone sensor parameters reset after power restore
547	Zone sensor parameters reset from system interruption
548	Zone humidity sensor error
551	Zone sensor lost communication
71511	Duct or filter blockage
71517	Indoor fan motor failure
E105	Wiring issue / communication problem
E114	AC line frequency / distortion problem
E115	Low secondary (24 VAC) voltage
E116	High secondary (24 VAC) voltage
E117	Poor ground
E120	Unresponsive device
E124	Thermostat lost communication to smart hub
E125	Control hardware problem
E126	Control internal communication problem
E130	Configuration jumper missing
E132	Device control software fault
E180	Outdoor temperature sensor problem
E201	Blower motor communication fault
E202	Blower motor and unit size mismatch
E203	Invalid unit size code
E292	Blower motor start fault
E295	Blower motor over temperature
E310	Discharge air temperature sensor problem
E312	Reduced airflow - indoor blower cutback
E345	Relay O failure
E346	Heat pump jumper not removed
E347	Indoor or EIM relay Y1 fault
E348	Indoor relay Y2 fault
E350	Electric heat not configured
E351	Electric heat stage 1 fault
E352	Electric heat stage 2 fault
E353	Electric heat stage 3 fault
E354	Electric heat stage 4 fault
E355	Electric heat stage 5 fault
E371	Float switch sensed open
E380	EIM interlock relay fault
E381	EIM interlock relay stuck
E382	EIM relay W1 fault
E420	EIM defrost out of cycle

Lennox Furnace error codes

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

Lennox Heat Pump error codes

Code	Meaning
10	Unknown device detected
11	Missing device
12	Indoor unit not detected
13	Duplicate comfort sensor ID
14	Too many devices of the same type
15	Parameter mismatch detected
20	Protocol upgrade required
21	Incompatible equipment detected
29	Over temperature protection (indoor above 90 F)
30	Low temperature protection (below 40 F)
31	Lost communication with device
32	Device resetting
34	Must program unit capacity for device
35	Incorrect operation of device
36	Heating when not requested
37	Cooling when not requested
38	Not heating when requested
39	No cooling when requested
40	HP heating lockout
41	Device control board replaced
610	Low room temperature detected
611	High room temperature detected
700	Thermostat temperature sensor problem
701	Thermostat temperature above limit
703	Thermostat humidity sensor problem
3000	Replace filter 1
3001	Replace filter 2
3002	Replace humidifier pad
3003	Replace UV bulb
3004	Maintenance reminder
3005	PureAir maintenance
65537	Missing base (smart hub)
65538	Missing base
65539	Thermostat lost connection or internal fault
65540	Missing HD wall display
65541	Firmware download failed
65542	Firmware update failed
65543	Firmware updated
65544	More than 8 thermostats in a group
65545	Cooling capacity alert
70001	IAQ internal sensor fault
70002	IAQ internal data fault
70003	IAQ side switch in OFF position
70004	Replace Smart IAQ monitor
70501	Smart room sensor internal data fault
70502	Smart room sensor internal fault
70503	Smart room sensor low battery
70504	Smart room sensor battery 1%
71001	Wireless extender internal data fault
71504	Suction temperature sensor incorrect reading
71505	Liquid temperature sensor incorrect reading
71506	Suction pressure sensor incorrect reading
71507	Liquid pressure sensor incorrect reading
71508	Outdoor temperature sensor incorrect reading
71509	Outdoor coil temperature sensor incorrect reading
71510	Indoor TXV failure
71512	High-pressure switch fault
71513	Low-pressure switch fault
71514	System low on refrigerant charge
71515	Reversing valve stuck
71516	Outdoor fan motor failure
71518	Compressor failed to start
71519	Incorrect reversing valve mode
71520	Blocked liquid line
71521	Outdoor TXV failure
80001	IAQ persistent poor air quality
80002	Unresponsive wireless device fault
80003	Lost communication with wireless device
80005	Unknown wireless device found
E105	Wiring issue / communication problem
E114	AC line frequency / distortion problem
E115	Low secondary (24 VAC) voltage
E120	Unresponsive device
E124	Thermostat lost communication to smart hub
E125	Control hardware problem
E126	Control internal communication problem
E132	Device control software fault
E180	Outdoor temperature sensor problem
E181	Outdoor suction pressure transducer fault
E182	Outdoor suction temperature sensor fault
E183	Outdoor liquid pressure sensor fault
E184	Outdoor liquid temperature sensor fault
E345	Relay O failure
E400	Compressor internal overload tripped
E401	Compressor long run cycle
E402	System pressure trip
E403	Compressor short-cycling
E404	Compressor rotor locked
E405	Compressor open circuit
E406	Compressor open start circuit
E407	Compressor open run circuit
E408	Compressor contactor welded
E409	Outdoor control low 24 VAC
E410	Open low pressure switch
E411	Low pressure switch strikes lockout
E412	Open high pressure switch
E413	High pressure switch strikes lockout
E414	High discharge line temperature
E415	High discharge line temperature strikes lockout
E416	Outdoor coil sensor faulty
E417	Discharge sensor faulty
E418	EIM W output hardware fault
E419	EIM W output hardware fault lockout
E420	EIM defrost out of cycle
E421	EIM W external miswire fault
E422	Compressor top cap switch open
E423	Inverter CT circuit fault (inverter code 40)
E424	Liquid line sensor faulty
E426	Excessive inverter alarms
E427	Inverter DC peak fault (inverter code 21)
E428	Inverter high main input current (inverter code 22)
E429	Inverter DC link low voltage (inverter code 23)
E430	Inverter compressor startup fail (inverter code 26)
E431	Inverter PFC fault (inverter code 27)
E432	Inverter DC link high voltage (inverter code 28)
E433	Inverter compressor over-current (inverter code 29)
E434	Inverter communication error to main control (inverter code 53)
E435	Inverter EEPROM checksum fault (inverter code 60)
E436	Inverter high heat-sink temperature (inverter code 62)
E437	Inverter heat-sink temperature sensor fault (inverter code 65)
E438	Inverter PFC input over-current (inverter code 73)
E440	Inverter compressor slowdown - high heat-sink temperature (inverter code 13)
E441	Inverter compressor slowdown - high compressor current (inverter code 14)
E442	Compressor top cap switch strikes lockout
E443	Unit code to inverter model mismatch
E444	Reversing valve relay or solenoid fault
E446	Low suction pressure fault
E600	Load shed event
E601	Low ambient operational lockout
Green & Red alternating FAST flash	Coil (defrost) sensor problem
Green & Red alternating SLOW flash	5-minute anti-short-cycle delay
Green & Red simultaneous FAST flash	Ambient (outdoor air) sensor problem
Green FAST flash / Red OFF	Discharge sensor lockout
Green FAST flash / Red ON	Discharge line temperature lockout
Green OFF / Red FAST flash	Discharge sensor fault
Green OFF / Red OFF	No power to the demand defrost control
Green OFF / Red ON	Low pressure lockout
Green OFF / Red SLOW flash	Low pressure fault
Green ON / Red OFF	High pressure lockout
Green ON / Red ON	Demand defrost control failure
Green SLOW flash / Red OFF	High pressure fault
Green SLOW flash / Red ON	Discharge line temperature fault

Find codes by Lennox model

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 error codes by equipment type

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Lennox Air Handler error codes

Lennox Furnace error codes

Lennox Heat Pump error codes

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