Lennox Heat Pump Error Codes — Meanings, Causes & Repair in Greater Vancouver

Seeing a fault code on your Lennox heat pump? This guide explains what the most common Lennox heat pump error codes mean, what’s safe to check yourself, and when to call HeatLand’s licensed HVAC technicians for Lennox heat pump repair across Metro Vancouver and the Fraser Valley.

The codes below apply across common Lennox models (Signature/iComfort, Elite, Merit). Meanings follow Lennox’s documented fault codes — always confirm against your unit’s own manual, because exact wording can vary by model, control board and firmware.

How to read Lennox heat pump error codes

iComfort/communicating systems display a 3-digit numeric Alert Code on the thermostat (e.g. S30/S40/E30 or iComfort Wi-Fi) and on the outdoor control's 7-segment display. Codes 4xx are outdoor-unit (air conditioner / heat pump) alerts. Variable-capacity inverter units also flash a separate 2-digit inverter flash code. Basic single-stage Merit heat pumps with a non-communicating thermostat have no on-screen code display.

Lennox heat pump error codes & what they mean

These are the Lennox heat pump codes our Greater Vancouver technicians see most. Anything marked pro repair involves electrical, refrigerant, sensor or control-board work that must be done by a licensed HVAC technician — never open the indoor/outdoor unit or refrigerant lines yourself.

Code	What it usually means	Common causes	Safe to check yourself
400	OD compressor internal overload tripped: thermostat demand Y1 is present but the compressor is not running; the compressor internal overload has tripped pro repair	Compressor internal overload (thermal) tripped from overheating; Loss of power to the outdoor unit; Compressor electrical fault	Confirm the outdoor unit breaker and outdoor disconnect are ON; Make sure the outdoor unit is clear of debris and has airflow; Try a single thermostat reset and allow the system several minutes to restart
401	OD compressor long run cycle: the compressor ran more than 18 hours continuously during a single demand, or system refrigerant pressure is low. Does not lock out the system pro repair	Low refrigerant charge / refrigerant leak; Dirty outdoor coil reducing capacity; Undersized or struggling system on an extreme-temperature day	Replace or clean the indoor air filter; Clear leaves, snow and debris from around the outdoor unit; Confirm thermostat setpoint and mode are reasonable for the weather
402	OD system pressure trip: either the discharge or suction pressure is out of limits, or the compressor has overloaded pro repair	Refrigerant charge problem (over or undercharge); Restricted airflow over indoor or outdoor coil; Compressor overload	Replace or clean the indoor air filter; Clear the outdoor unit of debris and ensure airflow; Allow the system to complete a few normal cycles after correcting airflow
403	OD compressor short-cycling: the compressor ran less than three minutes to satisfy a thermostat demand pro repair	Pressure switch tripping the compressor early; Refrigerant charge problem; Control or wiring fault causing premature shutoff	Replace or clean the indoor air filter; Clear the outdoor unit of obstructions; Confirm thermostat is not rapidly cycling the call (single reset)
404	OD compressor rotor locked: the compressor rotor is locked up (e.g. run capacitor short, seized bearings, or excessive liquid refrigerant). May need a hard start kit pro repair	Failed/shorted run capacitor; Seized compressor bearings; Liquid refrigerant flooding the compressor	Confirm the outdoor breaker and disconnect are ON; Allow the system to attempt a restart over a few normal cycles; Do not repeatedly force restarts — call a technician
405	OD compressor open circuit: the compressor circuit is open due to a power disconnection or an open fuse pro repair	Open fuse in the outdoor unit; Power disconnection to the compressor; Wiring fault	Check the outdoor unit breaker and outdoor disconnect are ON; Confirm power is reaching the outdoor unit
406	OD compressor open start circuit: the required current is not passing through the START current transformer pro repair	Start capacitor or start-circuit fault; Compressor start winding issue; Wiring/sensor problem at the outdoor control	Confirm the outdoor breaker and disconnect are ON; Allow the system to attempt a restart
407	OD compressor open run circuit: the required current is not passing through the RUN current transformer pro repair	Run capacitor or run-winding fault; Compressor electrical fault; Wiring/sensor problem at the outdoor control	Confirm the outdoor breaker and disconnect are ON; Allow the system to attempt a restart over a normal cycle
408	OD compressor contactor welded: the compressor is running continuously pro repair	Welded (stuck closed) compressor contactor; Control relay fault	If the outdoor unit runs nonstop, turn the system off at the thermostat and call a technician; Confirm thermostat mode is set correctly
409	OD control board low 24VAC: the secondary voltage for the outdoor control has fallen below 18VAC; if it continues 10 minutes the thermostat turns off the component pro repair	Low or failing transformer output; Loose or corroded low-voltage wiring; Excessive load on the 24VAC circuit	Confirm the outdoor breaker and disconnect are ON; Try a single power reset at the breaker; Do not open the control panel — 24VAC wiring requires a technician
410	OD open low-pressure switch: unit low pressure is below the required limit; the low-pressure switch has opened (system shuts down and restarts when pressure recovers) pro repair	Low refrigerant charge / refrigerant leak; Restricted airflow over the indoor coil; Outdoor coil iced over in heating	Replace or clean the indoor air filter; Clear snow and ice from around the outdoor unit; Confirm registers/vents are open and unobstructed
411	OD low-pressure switch strikes lockout: the low-pressure switch opened five times during one cooling or heating demand; the thermostat shuts down the outdoor unit pro repair	Low refrigerant charge / refrigerant leak; Restricted airflow over the indoor coil; Iced outdoor coil or stuck reversing valve in heating	Replace or clean the indoor air filter; Clear snow, ice and debris from the outdoor unit; Try one thermostat/breaker reset; if the lockout returns, call a technician
412	OD open high-pressure switch: unit high pressure is above the upper limit and the system shuts down. Check charge, condenser fan, blower, stuck reversing valve or clogged refrigerant filter pro repair	Dirty/blocked outdoor coil or condenser fan not running; Overcharge or restricted refrigerant filter; Stuck reversing valve or blower problem	Clean the outdoor unit and clear obstructions from airflow; Replace or clean the indoor air filter; Confirm the outdoor fan spins freely when off (visual only)
413	OD high-pressure switch strikes lockout: the high-pressure switch opened five times during one cooling demand; the thermostat shuts down the outdoor unit pro repair	Dirty outdoor coil or blocked airflow; Outdoor fan motor not running; Overcharge or refrigerant restriction	Clean the outdoor unit and remove debris blocking airflow; Replace or clean the indoor air filter; Try one breaker reset; if lockout returns, call a technician
414	OD high discharge-line temperature: the discharge line temperature is higher than the recommended upper limit of 279°F pro repair	Low refrigerant charge; Dirty coil / restricted airflow; Refrigerant restriction raising discharge temperature	Make sure the outdoor coil is clean and airflow is unobstructed; Replace or clean the indoor air filter
415	OD high discharge-line temperature strikes lockout: discharge line temperature exceeded 279°F for five strikes in a single demand; the unit locks out pro repair	Low refrigerant charge; Dirty coil / obstructed airflow at condenser; Refrigerant restriction	Clean the outdoor coil and clear obstructions to airflow; Replace or clean the indoor air filter; Correct airflow then cycle power once
416	OD coil sensor faulty: the outdoor coil temperature sensor is open, short-circuited, or out of range; the outdoor unit cannot perform proper defrost (system still heats/cools) pro repair	Failed outdoor coil temperature sensor; Loose or miswired sensor harness; Sensor out of calibration	No safe homeowner check beyond a single power reset — sensor diagnosis requires a technician
417	OD discharge sensor faulty: the outdoor discharge line temperature sensor is open, short-circuited, or out of range pro repair	Failed discharge line temperature sensor; Sensor leads in wrong harness pin positions; Damaged sensor cable assembly	No safe homeowner check beyond a single power reset — sensor diagnosis requires a technician
418	OD EIM W output hardware fault: a faulty W output circuit — the W terminal is energized while in cooling mode (possible stuck relay or external miswire) pro repair	Stuck-closed relay on the control; External wiring energizing the W terminal; Control board hardware fault	No safe homeowner check — W-output diagnosis requires opening low-voltage wiring; call a technician
419	OD EIM W output hardware fault lockout: the W output reported more than five strikes (alert 418); the system shuts down the outdoor unit pro repair	Repeated W-output / stuck-relay faults; Persistent external W-terminal miswire; Failed outdoor control	Try a single power recycle at the breaker; if it returns, call a technician
420	AH EIM defrost out of cycle: the heat pump defrost cycle has taken more than 20 minutes to complete. Applicable to a communicating indoor unit paired with a non-communicating heat pump pro repair	Outdoor coil heavily iced or restricted airflow; Reversing valve or defrost control problem; Defrost/coil sensor issue	Clear snow and ice buildup from around the outdoor unit; Make sure nothing blocks airflow to the outdoor coil; Try one thermostat reset and observe whether defrost completes
421	OD EIM W external miswire fault: the W output terminal on the outdoor unit is not wired correctly — voltage is sensed on W output when Y1 out is deactivated pro repair	Incorrect field wiring at the outdoor W terminal; Crossed thermostat/control wiring; External device backfeeding the W terminal	No safe homeowner check — wiring correction requires a technician
422	OD compressor top cap switch open: the compressor top cap (thermal) switch is exceeding its thermal limit. Check for stuck reversing valve or clogged refrigerant filter pro repair	Stuck reversing valve or clogged refrigerant filter; Condenser fan, TXV or indoor blower problem; Compressor overheating	Clean the outdoor unit and clear obstructions from airflow; Replace or clean the indoor air filter; Allow the unit to cool, then try one reset
423	OD inverter CT circuit fault: the inverter detected a current-transformer circuit issue; the outdoor control stops operation and starts the anti-short-cycle timer, locking out after 10 strikes within an hour (inverter flash code 40) pro repair	Inverter current-transformer / circuit fault; Inverter board hardware issue; Compressor electrical fault	For a service-urgent lockout, a single disconnect/restart of the outdoor unit may clear it; if it returns, call a technician
424	OD liquid line sensor faulty: the liquid line temperature sensor has malfunctioned (a valid temperature reading is lost during normal operation) pro repair	Failed liquid line temperature sensor; Loose or miswired sensor harness; Sensor out of range	No safe homeowner check beyond a single power reset — sensor diagnosis requires a technician
426	OD excessive inverter alarms: after 10 inverter faults within 60 consecutive minutes the control locks out (inverter flash codes 12–14 and 53) pro repair	Repeated inverter alarm conditions; Underlying refrigerant, airflow or electrical problem triggering frequent inverter faults; Inverter board fault	Disconnect power to the outdoor control and restart once; if the lockout returns, call a technician
427	OD inverter DC peak fault: the inverter detected a DC peak current condition (55A or higher); the outdoor unit stops and locks out after 10 occurrences within an hour (inverter flash code 21). On XP20/XP25 it may occur entering/exiting defrost pro repair	Inverter DC peak / compressor current fault; Compressor electrical fault; Older inverter software (pre serial 5817F) triggering during defrost	Disconnect and reconnect power to the outdoor control once; if it persists, call a technician
428	OD inverter high main input current: the inverter detected a high main input current condition; the outdoor unit stops and locks out after 10 occurrences within an hour (inverter flash code 22) pro repair	High main input current at the inverter; Compressor or electrical fault; Voltage supply issue	Disconnect power to the outdoor unit and restart once; if it returns, call a technician
429	OD inverter DC link low voltage: the inverter detected a low DC-link voltage condition (inverter flash code 23) pro repair	Low incoming supply voltage; Inverter DC-link / power-supply fault; Loose high-voltage connection	Confirm the outdoor breaker and disconnect are ON; Disconnect/restart the outdoor unit once; if it persists, call a technician
430	OD inverter compressor startup fail: the inverter could not start the compressor (inverter flash code 26) pro repair	Compressor failed to start / locked rotor; Inverter drive fault; Refrigerant pressure imbalance at startup	Disconnect power to the outdoor unit and restart once; if it returns, call a technician
431	OD inverter fault (inverter flash code 27) pro repair	Internal inverter fault; Compressor electrical fault	Disconnect power to the outdoor unit and restart once; if it persists, call a technician
432	OD inverter DC link high voltage: the inverter detected a high DC-link voltage condition (inverter flash code 28) pro repair	High incoming supply voltage; Inverter DC-link fault; Regenerative voltage during compressor deceleration	Disconnect power to the outdoor unit and restart once; if it persists, call a technician
433	OD inverter compressor over-current (inverter flash code 29); typically occurs during start-up. Updated outdoor-control software adds a 6-minute (cooling) / 11-minute (heating) delay after this code pro repair	Compressor over-current at startup; High system pressures; Inverter drive fault	Disconnect/reconnect power to the outdoor unit and restart the system once; if it persists, call a technician
434	OD communication error to main control (inverter flash code 53): communication is lost between the inverter and the outdoor main control pro repair	Internal inverter-to-control communication wiring fault; Inverter or control board fault; Loose connector	Cycle the outdoor unit power off and back on, then the indoor power, once; if it returns, call a technician
435	OD inverter EEPROM checksum fault (inverter flash code 60) pro repair	Inverter EEPROM / memory fault; Inverter board hardware failure	Disconnect power to the outdoor unit and restart once; if it persists, call a technician
436	OD inverter high heat-sink temperature (inverter flash code 62): the inverter heat sink temperature is too high pro repair	Restricted airflow / debris on the inverter heat sink; Loose heat-sink mounting screws; High outdoor ambient or high load	Clear debris and obstructions from around the outdoor unit; Do not open the control panel; a single power reset is the only safe homeowner step
437	OD inverter heat-sink temperature sensor fault (inverter flash code 65) pro repair	Failed inverter heat-sink temperature sensor; Sensor wiring fault; Inverter board fault	Disconnect/reconnect power to the outdoor unit once; sensor diagnosis requires a technician
438	OD inverter over-current (inverter flash code 73) pro repair	Inverter over-current condition; Compressor electrical fault; High system pressures / load	Allow the system to clear automatically after the inverter powers down; if it persists, call a technician
440	OD inverter compressor slowdown — high heat-sink temperature (inverter flash code 13): informational; the inverter briefly reduces compressor speed when the heat sink gets hot. Frequent occurrence (especially at low outdoor temps) suggests heat-sink debris or airflow obstruction DIY-checkable	Debris reducing heat transfer at the inverter heat sink; Airflow obstruction across the heat sink; High operating pressures	Clear debris and obstructions from around the outdoor unit; Often clears on its own — monitor; if frequent, call a technician
441	OD inverter compressor slowdown — high compressor current (inverter flash code 14): informational; the inverter reduces compressor speed and ramp rate because compressor current is approaching its limit. Typically occurs at startup. Usually no service required DIY-checkable	Normal current limiting at startup; System running at high pressures; Other high-pressure or load conditions	Usually no action needed — it clears when the condition ends; If it appears with other alert codes, call a technician
442	OD compressor top cap switch strikes lockout: the top cap (thermal protection) switch opened five times within one hour and the outdoor unit is locked out pro repair	Compressor overheating / thermal protection; Stuck reversing valve or refrigerant restriction; Condenser fan, TXV or blower problem	Clean the outdoor unit and clear airflow obstructions; Replace or clean the indoor air filter; Allow the unit to cool, then try one disconnect/restart; if lockout returns, call a technician
443	OD MUC unit code to inverter model mismatch: the Lennox variable-capacity unitary (outdoor) control has the wrong appliance unit-size code selected pro repair	Incorrect unit-size configuration on the outdoor control; Replacement inverter not matched to unit size; Control board reconfiguration needed	No safe homeowner check — unit-size configuration must be corrected by a technician
444	HP reversing valve relay or solenoid fault: a relay failure on the reversing-valve (O) output. Verified by a call for heat pump heating and checking for 24VAC out from the O terminal. Resolution is to replace the outdoor unit control board pro repair	Failed reversing-valve relay on the outdoor control board; Reversing-valve solenoid (O) circuit fault; Outdoor control board failure	No safe homeowner check — relay/solenoid diagnosis and board replacement require a licensed technician
446	OD low suction pressure fault: suction pressure below 40 psig in operation; starts as service soon and escalates to service urgent, stopping the system after five strikes in a single demand. Recommends replacing the low-pressure switch pro repair	Low refrigerant charge / refrigerant leak; Restricted airflow over indoor coil; Faulty low-pressure switch	Replace or clean the indoor air filter; Clear snow, ice and debris from the outdoor unit; Confirm vents/registers are open; if the fault returns, call a technician

Lennox heat pump codes we get called for most

409 — OD control board low 24VAC: the secondary voltage for the outdoor control has fallen below 18VAC; if it continues 10 minutes the thermostat turns off the component

Typical causes include low or failing transformer output; loose or corroded low-voltage wiring; excessive load on the 24vac circuit. A few things are safe to check yourself (Confirm the outdoor breaker and disconnect are ON; Try a single power reset at the breaker), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 409 on Lennox heat pumps throughout Greater Vancouver.

416 — OD coil sensor faulty: the outdoor coil temperature sensor is open, short-circuited, or out of range; the outdoor unit cannot perform proper defrost (system still heats/cools)

Typical causes include failed outdoor coil temperature sensor; loose or miswired sensor harness; sensor out of calibration. A few things are safe to check yourself (No safe homeowner check beyond a single power reset — sensor diagnosis requires a technician), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 416 on Lennox heat pumps throughout Greater Vancouver.

400 — OD compressor internal overload tripped: thermostat demand Y1 is present but the compressor is not running; the compressor internal overload has tripped

Typical causes include compressor internal overload (thermal) tripped from overheating; loss of power to the outdoor unit; compressor electrical fault. A few things are safe to check yourself (Confirm the outdoor unit breaker and outdoor disconnect are ON; Make sure the outdoor unit is clear of debris and has airflow), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 400 on Lennox heat pumps throughout Greater Vancouver.

401 — OD compressor long run cycle: the compressor ran more than 18 hours continuously during a single demand, or system refrigerant pressure is low. Does not lock out the system

Typical causes include low refrigerant charge / refrigerant leak; dirty outdoor coil reducing capacity; undersized or struggling system on an extreme-temperature day. A few things are safe to check yourself (Replace or clean the indoor air filter; Clear leaves, snow and debris from around the outdoor unit), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 401 on Lennox heat pumps throughout Greater Vancouver.

Is it safe to keep running a Lennox heat pump with a fault code?

Most Lennox fault codes put the heat pump into a safe lockout for a reason. You can safely check the basics — the power and breaker on, the indoor filter clean, the outdoor unit clear of snow, ice and debris, and the remote set to the correct mode and temperature — and restart it once. If the code returns, stop and book a technician. Mini-split and heat-pump faults are electrical and refrigerant issues — never open the indoor or outdoor unit, and never touch the wiring or refrigerant lines yourself.

Lennox heat pump repair across Greater Vancouver

HeatLand has kept Metro Vancouver homes comfortable since 1999. Our licensed, insured technicians diagnose and repair Lennox heat pumps — sensor and thermistor errors, indoor-to-outdoor communication faults, fan and compressor problems, refrigerant and pressure faults, and control-board issues — including units we didn’t install. We’re brand-agnostic, carry common parts, and give you an honest repair-or-replace assessment with upfront pricing and a free estimate. If your heat pump is beyond economical repair, we’ll show you right-sized, high-efficiency replacement options and the rebates available.

Need it sorted fast? We offer priority emergency heat pump repair. Explore Heat Pump services, browse all heat pump error codes by brand, or request a free estimate.

Lennox heat pump service areas

We repair Lennox heat pumps across Greater Vancouver and the Fraser Valley, including:

Lennox heat pump error code FAQs

Can I reset my Lennox heat pump myself?

Yes — power-cycling the unit at the breaker once is generally safe after you’ve checked the basics (the power and breaker on, the indoor filter clean, the outdoor unit clear of snow, ice and debris, and the remote set to the correct mode and temperature). If the heat pump faults again right away, stop and note the code — repeatedly resetting a heat pump that keeps locking out can mask a real problem. Call a licensed HVAC technician to diagnose it.

What does the Lennox 400 code mean?

On Lennox heat pumps, 400 commonly indicates od compressor internal overload tripped: thermostat demand y1 is present but the compressor is not running; the compressor internal overload has tripped. Confirm the exact meaning against your unit’s manual — codes can vary by model and series — and have the repair done by a licensed HVAC technician. HeatLand fixes this across Greater Vancouver.

Do you repair Lennox heat pumps you didn’t install?

Yes. HeatLand is brand-agnostic and services Lennox heat pumps throughout Metro Vancouver and the Fraser Valley, whether or not we installed the unit. We carry common parts and give you an honest repair-or-replace assessment.

Is a Lennox heat pump fault code an emergency?

No heat in cold weather or no cooling in a heat wave is worth treating urgently. For priority heat pump repair across Greater Vancouver, call (604) 330-3812.

How much does Lennox heat pump repair cost in Vancouver?

It depends on the fault. Many Lennox codes point to sensors, thermistors, capacitors or control parts — usually far cheaper than replacement. We give upfront pricing and a free estimate before any work.

More Heat Pump error-code guides

HeatLand repairs every major heat pump brand across Greater Vancouver. More brand guides: