Lennox Heat Pump Error Codes

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

142 Lennox Heat Pump 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 Heat Pump error codes list

Code	Meaning	Severity
10	Unknown device detected	High
11	Missing device	High
12	Indoor unit not detected	High
13	Duplicate comfort sensor ID	High
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
610	Low room temperature detected	High
611	High 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
65545	Cooling capacity alert	Medium
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
71504	Suction temperature sensor incorrect reading	Low
71505	Liquid temperature sensor incorrect reading	Low
71506	Suction pressure sensor incorrect reading	Low
71507	Liquid pressure sensor incorrect reading	Low
71508	Outdoor temperature sensor incorrect reading	Low
71509	Outdoor coil temperature sensor incorrect reading	Low
71510	Indoor TXV failure	Medium
71512	High-pressure switch fault	High
71513	Low-pressure switch fault	Medium
71514	System low on refrigerant charge	Medium
71515	Reversing valve stuck	High
71516	Outdoor fan motor failure	High
71518	Compressor failed to start	High
71519	Incorrect reversing valve mode	High
71520	Blocked liquid line	High
71521	Outdoor TXV 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	Wiring issue / communication problem	High
E114	AC line frequency / distortion problem	High
E115	Low secondary (24 VAC) voltage	High
E120	Unresponsive device	Medium
E124	Thermostat lost communication to smart hub	High
E125	Control hardware problem	High
E126	Control internal communication problem	High
E132	Device control software fault	High
E180	Outdoor temperature sensor problem	Medium
E181	Outdoor suction pressure transducer fault	Medium
E182	Outdoor suction temperature sensor fault	Medium
E183	Outdoor liquid pressure sensor fault	Medium
E184	Outdoor liquid temperature sensor fault	Medium
E345	Relay O failure	High
E400	Compressor internal overload tripped	Medium
E401	Compressor long run cycle	Low
E402	System pressure trip	Medium
E403	Compressor short-cycling	Medium
E404	Compressor rotor locked	High
E405	Compressor open circuit	High
E406	Compressor open start circuit	High
E407	Compressor open run circuit	High
E408	Compressor contactor welded	High
E409	Outdoor control low 24 VAC	Medium
E410	Open low pressure switch	Low
E411	Low pressure switch strikes lockout	High
E412	Open high pressure switch	Low
E413	High pressure switch strikes lockout	High
E414	High discharge line temperature	Medium
E415	High discharge line temperature strikes lockout	High
E416	Outdoor coil sensor faulty	Medium
E417	Discharge sensor faulty	Medium
E418	EIM W output hardware fault	Medium
E419	EIM W output hardware fault lockout	High
E420	EIM defrost out of cycle	Medium
E421	EIM W external miswire fault	High
E422	Compressor top cap switch open	Medium
E423	Inverter CT circuit fault (inverter code 40)	High
E424	Liquid line sensor faulty	Medium
E426	Excessive inverter alarms	High
E427	Inverter DC peak fault (inverter code 21)	High
E428	Inverter high main input current (inverter code 22)	High
E429	Inverter DC link low voltage (inverter code 23)	High
E430	Inverter compressor startup fail (inverter code 26)	High
E431	Inverter PFC fault (inverter code 27)	High
E432	Inverter DC link high voltage (inverter code 28)	High
E433	Inverter compressor over-current (inverter code 29)	High
E434	Inverter communication error to main control (inverter code 53)	High
E435	Inverter EEPROM checksum fault (inverter code 60)	High
E436	Inverter high heat-sink temperature (inverter code 62)	High
E437	Inverter heat-sink temperature sensor fault (inverter code 65)	High
E438	Inverter PFC input over-current (inverter code 73)	High
E440	Inverter compressor slowdown - high heat-sink temperature (inverter code 13)	Low
E441	Inverter compressor slowdown - high compressor current (inverter code 14)	Low
E442	Compressor top cap switch strikes lockout	High
E443	Unit code to inverter model mismatch	High
E444	Reversing valve relay or solenoid fault	High
E446	Low suction pressure fault	High
E600	Load shed event	Low
E601	Low ambient operational lockout	Low
Green & Red alternating FAST flash	Coil (defrost) sensor problem	High
Green & Red alternating SLOW flash	5-minute anti-short-cycle delay	Low
Green & Red simultaneous FAST flash	Ambient (outdoor air) sensor problem	Medium
Green FAST flash / Red OFF	Discharge sensor lockout	Medium
Green FAST flash / Red ON	Discharge line temperature lockout	Medium
Green OFF / Red FAST flash	Discharge sensor fault	Low
Green OFF / Red OFF	No power to the demand defrost control	Medium
Green OFF / Red ON	Low pressure lockout	High
Green OFF / Red SLOW flash	Low pressure fault	Medium
Green ON / Red OFF	High pressure lockout	High
Green ON / Red ON	Demand defrost control failure	High
Green SLOW flash / Red OFF	High pressure fault	Medium
Green SLOW flash / Red ON	Discharge line temperature fault	Low

Find codes by Lennox model

Lennox EL16KP1

EL16KP1-018 EL16KP1-024 EL16KP1-030 EL16KP1-036 EL16KP1-042 EL16KP1-048 EL16KP1-060

Lennox ML14KP1

ML14KP1-018 ML14KP1-024 ML14KP1-030 ML14KP1-036 ML14KP1-042 ML14KP1-048 ML14KP1-060

Heat Pump troubleshooting guides

Heat pump not defrosting / outdoor coil iced - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Low capacity or abnormal pressures - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Outdoor unit will not start - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Two horizontal lines on display (soft disable) - Lennox communicating Lennox · Heat Pump

Lennox Heat Pump specs and setup data

Outdoor unit clearances Lennox · Heat Pump · Lennox EL16KP1
Outdoor unit clearances Lennox · Heat Pump · Lennox ML14KP1

Homeowner help: AC Running But Not Cooling: What to Check · Heat Pump Not Heating or Blowing Cold Air

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.