HVACopediaLennox heat pump error codes
142 codes with causes and test steps, plus 2 settings tables.
| 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 |
Settings and specs
Outdoor unit clearances
Clearances · Lennox EL16KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 30 in. (762 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Between two units | 24 in. (610 mm) |
| Top | 48 in. (1219 mm) |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
- Increase clearances where snow/ice can fall on the unit or air can recirculate (decks, tight spaces).
- Wind barrier, if used, 24 in. (610 mm) from the unit.
Source: Lennox EL16KP1 Unit Information 100195 (03-2025), Installation Clearances p.6, Roof Mounting p.14
Outdoor unit clearances
Clearances · Lennox ML14KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 24 in. (610 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Top | 48 in. (1219 mm) |
| Multi-unit, between units | 12 in. (305 mm) minimum [18 in. (457 mm) recommended] or 24 in. (610 mm) depending on layout |
| Multi-unit service side | 24 in. (610 mm) recommended |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
- Multi-unit spacing (A/B) depends on single row, wall or multiple rows layout - see figure on p.7.
- Increase clearances where snow/ice can fall on the unit or air can recirculate.
Source: Lennox ML14KP1 Unit Information 100108 (03-2026), Installation Clearances p.7, Roof Mounting p.14
Models covered
EL16KP1-018EL16KP1-024EL16KP1-030EL16KP1-036EL16KP1-042EL16KP1-048EL16KP1-060ML14KP1-018ML14KP1-024ML14KP1-030ML14KP1-036ML14KP1-042ML14KP1-048ML14KP1-060
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.