Lennox Air Conditioner Error Codes — Meanings, Causes & Repair in Greater Vancouver

Seeing a fault code on your Lennox air conditioner? This guide explains what the most common Lennox air conditioner error codes mean, what’s safe to check yourself, and when to call HeatLand’s licensed HVAC technicians for Lennox air conditioner 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 air conditioner error codes

Lennox communicating (iComfort / S30 / S40 Smart) systems show numeric Alert Codes on the thermostat's Dealer/Service Support Center notifications screen and, on units with a seven-segment outdoor control, on the outdoor board display. AC-related faults are reported by the outdoor unit (OD = AC or HP) and by the communication/voltage layer. Codes below are from the official Lennox Residential Communicating Systems Alert Code Guide V2 (doc 100017, 3/2023). Heat-pump-only, inverter-flash, furnace-heating, zoning and PureAir codes are excluded as they do not apply to a standard central air conditioner. Non-communicating (Merit / older Elite) air conditioners paired with a non-communicating thermostat do NOT display alert codes.

Lennox air conditioner error codes & what they mean

These are the Lennox air conditioner 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
10	Unknown device detected. The thermostat (not in commissioning mode) found an unknown/new communicating device on the system and soft-disabled it (double horizontal lines on the outdoor seven-segment display) pro repair	A communicating device added after the original setup; System not re-commissioned after equipment change	Confirm thermostat is set correctly; Note the code and contact your dealer to re-commission the system
11	Missing device. The thermostat cannot find a previously installed communicating system component pro repair	Loss of low-voltage connection to a unit; Failed control board; Component is not Lennox-communicating compatible	Try a single power cycle at the thermostat; Note the code and contact your dealer if it persists
30	Low temperature protection. The thermostat will not allow cooling to begin until it senses an indoor temperature above 40 deg F DIY-checkable	Indoor temperature below 40 deg F; Cooling equipment stuck ON; Inaccurate thermostat temperature sensor	Confirm indoor temperature is above 40 deg F; Switch system mode to Heat to warm the space if it is genuinely cold
31	Lost communication with device. The applicable component (indoor unit, equipment interface module, damper control module or outdoor unit) has not communicated with the thermostat for more than three minutes pro repair	Loose or damaged low-voltage wiring; Tripped condensate float switch (wired in series with R) from a clogged condensate drain; Freezestat tripped	If the drain line is clogged or there is water around the indoor unit, clear standing water and have the condensate drain cleaned; Try a single power cycle; Set thermostat to Cool if cooling is wanted
105	Wiring issue / communication problem. Low-voltage wiring between system components has been compromised and a device cannot communicate pro repair	Loose terminal connections; Incorrectly wired or poorly spliced connections; Low voltage routed too close to high-voltage wiring (inductive noise)	Note the code; this requires inspection of low-voltage wiring; Do not open control panels; contact your dealer
110	Low AC line voltage. The component's incoming AC line voltage is too low (may appear during a brownout or when line voltage is below its designed operating value) pro repair	Utility brownout / sag; Line voltage below nameplate rating; Loose or undersized line-voltage wiring	Check whether other appliances/lights show signs of low power; Confirm the breaker/disconnect is fully ON; Contact dealer or electrician if it persists
112	No ground connection. The reporting component cannot find earth ground; the thermostat will shut down the system pro repair	Missing or broken equipment ground; Wiring/grounding fault at the unit or panel	Confirm the breaker/disconnect is ON; This is an electrical fault; contact a licensed dealer or electrician
113	High AC line voltage. Incoming line voltage is higher than the nameplate rating pro repair	Utility over-voltage condition; Incorrect supply voltage / wiring	Contact a licensed dealer or electrician to verify supply voltage
400	OD compressor internal overload tripped. The compressor's internal overload has opened: thermostat demand (Y1) is present but the compressor is not running pro repair	Compressor overheating; Loss of power to the unit; Electrical/mechanical compressor stress	Confirm the outdoor disconnect and breaker are ON; Make sure the outdoor condenser coil is clear of debris and has airflow; Allow the unit to cool, then try a single restart
401	OD compressor long run cycle (information). The compressor ran more than 18 hours continuously during a single cooling demand, or system refrigerant pressure is low. Does not lock out the system pro repair	Low refrigerant charge / leak; Undersized system or extreme load; Dirty coil or filter reducing capacity	Clean or replace the indoor air filter; Clear the outdoor condenser of debris and vegetation; Note the code and contact your dealer (possible low refrigerant)
402	OD system pressure trip. Either the discharge or suction pressure is out of limits, or the compressor has overloaded pro repair	Dirty condenser or evaporator coil; Low or overcharged refrigerant; Restricted airflow	Clean or replace the air filter; Clear the outdoor condenser coil of dirt and debris; Note the code and contact your dealer
403	OD compressor short-cycling. The compressor ran for less than three minutes to satisfy a thermostat demand pro repair	Pressure switch tripping; Thermostat or wiring issue; Refrigerant or airflow problem causing rapid cutout	Clean or replace the air filter; Make sure the outdoor unit has clear airflow; Note the code and contact your dealer
404	OD compressor rotor locked. The compressor rotor is locked due to a shorted run capacitor, seized bearings, or excessive liquid refrigerant pro repair	Failed/shorted run capacitor; Seized compressor bearings; Liquid refrigerant flooding the compressor	Confirm the outdoor disconnect/breaker is ON; Do not attempt repairs; this is an internal compressor fault requiring a technician
405	OD compressor open circuit. The compressor circuit is open due to a power disconnection or an open fuse pro repair	Power disconnected to the outdoor unit; Blown/open fuse; Open wiring to the compressor	Confirm the outdoor disconnect and breaker are ON; If power is on and the code persists, contact your dealer
406	OD compressor open start circuit. The required current is not passing through the START current transformer pro repair	Failed start component/capacitor; Open or loose start-circuit wiring; Compressor start winding fault	Confirm power to the outdoor unit; This is an internal electrical fault; contact your dealer
407	OD compressor open run circuit. The required current is not passing through the RUN current transformer pro repair	Open or loose run-circuit wiring; Failed run capacitor; Compressor run winding fault	Confirm power to the outdoor unit; This is an internal electrical fault; contact your dealer
408	OD compressor contactor welded. The compressor is running continuously (contactor stuck closed) pro repair	Welded/stuck contactor; Contactor failure	If the outdoor unit runs nonstop, turn the system Off at the thermostat and contact your dealer; Do not open the electrical panel
409	OD control board low 24VAC. The outdoor unit's secondary (24VAC) control voltage has fallen below 18VAC; if it continues for 10 minutes the thermostat turns the unit off pro repair	Overloaded or weak transformer; Too many devices on the secondary circuit; Low-voltage wiring fault	Note the code; this requires checking transformer/low-voltage circuit; Contact your dealer
410	OD open low pressure switch (information). The unit's low pressure is below the required limit and the low-pressure switch has opened (system shuts down, restarts when pressure recovers) pro repair	Low refrigerant charge / leak; Restricted airflow or dirty coil; Cold operating conditions	Clean or replace the air filter; Clear the outdoor condenser of debris; Note the code and contact your dealer (possible refrigerant issue)
411	OD low pressure switch strikes lockout. The low-pressure switch has opened five times during one cooling (or heating) demand; the thermostat shuts down the outdoor unit and locks it out pro repair	Low refrigerant charge / leak (most common); Restricted airflow or dirty filter/coil; Failed low-pressure switch	Clean or replace the air filter; Clear the outdoor condenser coil of dirt and debris; Note the code and contact your dealer; this commonly indicates low refrigerant
412	OD open high pressure switch (information). The unit's high pressure is above the upper limit and the high-pressure switch has opened (system shuts down, restarts when the switch closes) pro repair	Dirty/blocked outdoor condenser coil; Failed condenser fan motor; Overcharge	Make sure the outdoor unit is clean and unobstructed; Confirm the outdoor fan spins when the unit runs (visually, from outside); Clean or replace the air filter
413	OD high pressure switch strikes lockout. The high-pressure switch has opened five times during one cooling demand; the thermostat shuts down and locks out the outdoor unit pro repair	Dirty condenser coil or blocked airflow; Failed outdoor fan; Overcharge / refrigerant issue	Clear dirt and debris blocking airflow to the outdoor unit; Confirm the outdoor fan is running (visually); Clean or replace the air filter
414	OD high discharge line temperature. Discharge line temperature is higher than the recommended upper limit of 279 deg F pro repair	Dirty coil or restricted airflow; Low refrigerant charge; Operating outside design pressures	Make sure the outdoor coil is clean and airflow is unobstructed; Clean or replace the air filter; Note the code and contact your dealer
415	OD high discharge line temperature strikes lockout. Discharge line temperature has exceeded 279 deg F five times during a single demand; the unit locks out pro repair	Dirty coil or blocked airflow; Low refrigerant charge; Improper charge / system pressures	Make sure the outdoor coil is clean and airflow is clear; Clean or replace the air filter; Note the code and contact your dealer
416	OD coil sensor faulty. The outdoor coil temperature sensor is open, short-circuited or reading out of range. The unit may still cool but defrost/demand temperature functions are affected pro repair	Disconnected or damaged coil sensor harness; Failed outdoor coil sensor; Sensor out of calibration	Note the code and contact your dealer for sensor inspection/replacement
417	OD discharge sensor faulty. The outdoor discharge line temperature sensor is open, short-circuited or out of range (escalates from service-soon to service-urgent after 10 minutes) pro repair	Discharge sensor leads in wrong harness pin positions; Failed discharge sensor or cable assembly	Note the code and contact your dealer for sensor inspection/replacement
418	OD EIM W output hardware fault. A faulty W (auxiliary/heat) output circuit: the W terminal is energized while in cooling mode (possible stuck-closed relay) pro repair	Stuck-closed relay on the outdoor/equipment interface control; External wiring energizing the W terminal	Note the code and contact your dealer; do not open the control
419	OD EIM W output hardware fault lockout. The W output (see code 418) has reported more than five strikes; the system shuts down the outdoor unit pro repair	Repeated stuck-relay / W-output fault on the outdoor control	Note the code and contact your dealer; may require replacing the outdoor control
421	OD EIM W external miswire fault. The W output terminal on the outdoor unit is not wired correctly; voltage is sensed on the W output when Y1 out is deactivated pro repair	Incorrect field wiring on the outdoor unit W terminal	Note the code and contact your dealer to correct wiring
422	OD compressor top cap switch open. The compressor top-cap thermal switch has exceeded its limit pro repair	Failed condenser fan motor; TXV or indoor blower problem; Stuck reversing valve or clogged refrigerant filter	Confirm the outdoor fan runs (visually); Make sure the outdoor unit airflow is clear; Clean or replace the air filter
442	OD compressor top cap switch strikes lockout. The compressor thermal protection (top cap) switch opened five times within one hour; the outdoor unit stops pro repair	Compressor overheating; Airflow restriction or fan failure; Refrigerant problem	Clear the outdoor condenser of debris; Confirm the outdoor fan runs (visually); Clean or replace the air filter
446	OD low suction pressure fault. Suction pressure below 40 psig in operation; escalates from service-soon to service-urgent and stops the unit after five strikes in a single demand pro repair	Low refrigerant charge / leak; Restricted airflow / dirty filter or coil; Failed low pressure switch/sensor	Clean or replace the air filter; Clear the outdoor condenser of debris; Note the code and contact your dealer (possible refrigerant issue)
601	OD unit low ambient operational lockout (information). The outdoor unit has cycled off on low-temperature protection; it will not operate at or below 4 deg F outdoor ambient (and locks out by 15 deg F if running). Normal protective behavior in cold weather DIY-checkable	Outdoor ambient temperature too low for AC/heat-pump operation	No action needed in cold weather; clears automatically when temperatures rise; If it appears in warm weather, note the code and contact your dealer

Lennox air conditioner codes we get called for most

10 — Unknown device detected. The thermostat (not in commissioning mode) found an unknown/new communicating device on the system and soft-disabled it (double horizontal lines on the outdoor seven-segment display)

Typical causes include a communicating device added after the original setup; system not re-commissioned after equipment change. A few things are safe to check yourself (Confirm thermostat is set correctly; Note the code and contact your dealer to re-commission the system), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 10 on Lennox air conditioners throughout Greater Vancouver.

416 — OD coil sensor faulty. The outdoor coil temperature sensor is open, short-circuited or reading out of range. The unit may still cool but defrost/demand temperature functions are affected

Typical causes include disconnected or damaged coil sensor harness; failed outdoor coil sensor; sensor out of calibration. A few things are safe to check yourself (Note the code and contact your dealer for sensor inspection/replacement), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 416 on Lennox air conditioners throughout Greater Vancouver.

31 — Lost communication with device. The applicable component (indoor unit, equipment interface module, damper control module or outdoor unit) has not communicated with the thermostat for more than three minutes

Typical causes include loose or damaged low-voltage wiring; tripped condensate float switch (wired in series with r) from a clogged condensate drain; freezestat tripped. A few things are safe to check yourself (If the drain line is clogged or there is water around the indoor unit, clear standing water and have the condensate drain cleaned; Try a single power cycle), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 31 on Lennox air conditioners throughout Greater Vancouver.

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

Typical causes include low refrigerant charge / leak; undersized system or extreme load; dirty coil or filter reducing capacity. A few things are safe to check yourself (Clean or replace the indoor air filter; Clear the outdoor condenser of debris and vegetation), but the repair is a licensed HVAC technician job. HeatLand diagnoses and fixes 401 on Lennox air conditioners throughout Greater Vancouver.

Is it safe to keep running a Lennox air conditioner with a fault code?

Most Lennox fault codes put the air conditioner into a safe lockout for a reason. You can safely check the basics — the power and breaker on, the air filter clean, the outdoor condenser clear of debris, and the thermostat set to cool — and restart it once. If the code returns, stop and book a technician. Air-conditioner faults are electrical and refrigerant issues — never open the unit or touch the wiring or refrigerant lines yourself.

Lennox air conditioner repair across Greater Vancouver

HeatLand has kept Metro Vancouver homes comfortable since 1999. Our licensed, insured technicians diagnose and repair Lennox air conditioners — sensor and thermistor errors, 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 air conditioner 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 air conditioner repair. Explore Air Conditioner services, browse all air conditioner error codes by brand, or request a free estimate.

Lennox air conditioner service areas

We repair Lennox air conditioners across Greater Vancouver and the Fraser Valley, including:

Lennox air conditioner error code FAQs

Can I reset my Lennox air conditioner 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 air filter clean, the outdoor condenser clear of debris, and the thermostat set to cool). If the air conditioner faults again right away, stop and note the code — repeatedly resetting a air conditioner that keeps locking out can mask a real problem. Call a licensed HVAC technician to diagnose it.

What does the Lennox 31 code mean?

On Lennox air conditioners, 31 commonly indicates lost communication with device. the applicable component (indoor unit, equipment interface module, damper control module or outdoor unit) has not communicated with the thermostat for more than three minutes. 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 air conditioners you didn’t install?

Yes. HeatLand is brand-agnostic and services Lennox air conditioners 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 air conditioner fault code an emergency?

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

How much does Lennox air conditioner 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 Air Conditioner error-code guides

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