Lennox Air Conditioner Error Codes List: Every Code Explained
Every Lennox air conditioner error code we could verify against Lennox's own documentation — what it means, what usually causes it, and the first fix to try — in one table. Codes differ between model series, so treat your model's tech sheet as the final word.
How to read and retrieve error codes on Lennox air conditioners
A communicating Lennox reports in two places at once, and a good diagnosis uses both. At the thermostat, alerts appear as numbers with priority: critical alerts show on the Home screen and in both the homeowner and installer alert buttons, while minor and moderate alerts live only behind the installer button — so a system can be carrying a moderate 410 or 412 that the homeowner screen never mentioned. At the outdoor unit, the A175 main control repeats the verdict in hardware: DS11 (green) and DS14 (red) flash combinations on LED models (slow flash for the 410/412 faults, solid for their lockouts, fast flash for the 415 lockout and the 417 sensor fault), or the number itself on 7-segment models. The A177 fan-motor control is a third, separate witness with its own tiny vocabulary — fast flash with no pause means its PWM feedback does not match the target RPM, constant ON means a CRC failure, and both end at fan-motor-control replacement. Read the thermostat number AND the outdoor display before resetting anything: the strike-based families (410→411, 412→413, 414→415) are a story told in sequence, and the first code of the sequence is the one that names the cause.
The complete Lennox air conditioner error code list
| Code | Meaning | Most likely cause | First fix |
|---|---|---|---|
| 400 | Compressor internal overload trip (iComfort alert, outdoor unit; XC21 Table 10 'Compressor Internal Overload Trip', DS15 yellow OFF / DS13 red ON) — a Y1 thermostat demand is present but the compressor is not running because its internal overload/protector is open | Lennox's XC21 cause list: compressor protector open, outdoor unit power disconnect open, unit breaker or fuse open, a broken wire or connector not making contact, a low or high pressure switch open if present, or the compressor contactor failed to close | Check power to the unit first (disconnect, breaker, fuses), then contactor and wiring — technician work beyond the disconnect. Clears after current is sensed in both RUN and START sensors for at least 2 seconds, or after service is removed, or after a power reset (Lennox's stated clearing) |
| 401 | Compressor long run time (iComfort alert, outdoor unit) — the compressor ran more than 18 hours continuously to satisfy a single thermostat demand, or system refrigerant pressure is low. The alert will not lock the system out | Lennox's XC21 cause list: low refrigerant charge, the evaporator blower not running, a frozen evaporator coil, a faulty metering device, a dirty condenser coil, a liquid-line restriction (blocked filter drier), or a malfunctioning thermostat. On two-stage units the unit runs in low speed (LED controls) or displays the code and continues in high speed (7-segment controls) while 401 stands | Owner checks: filter, indoor blower actually running, registers/ducts unblocked. Charge, metering-device and pressure diagnosis are certified-technician work. Clears after 30 consecutive normal run cycles or after a power reset. Lennox also notes 401 monitors low-pressure-switch trips — a 401 with pressure trips is a charge story until proven otherwise |
| 402 | System pressure trip (iComfort alert, outdoor unit) — either the discharge or suction pressure level is out of limits, or the compressor has overloaded; the XC21 table words it as the compressor protector open or missing supply power to the compressor | Lennox's XC21 cause list: high head pressure (check the high-pressure switch if present, an overcharged system, non-condensables), a condenser coil with poor air circulation (dirty, blocked, damaged), the condenser fan not running (capacitor, wiring, motor failure or blockage), or substantial return-air duct leakage | Clean the outdoor coil and prove the outdoor fan. Pressure and charge verification against the unit charging charts is certified-technician work. Clears after four consecutive normal compressor run cycles, or after a power reset |
| 403 | Compressor short cycling (iComfort alert, outdoor unit) — the compressor ran for less than 3 minutes to satisfy a thermostat demand | Lennox's XC21 cause list: an intermittent thermostat demand signal, a defective time-delay relay or A175 main control, or — if a high-pressure switch is present — the high-pressure fault family behind alert 412 | Technician work: thermostat signal and wiring, control diagnosis, and the pressure story if 412 keeps company with it. Clears after four consecutive normal run cycles or after a power reset |
| 404 | Compressor locked rotor (iComfort alert, outdoor unit) — the compressor rotor is locked up | Lennox's listed causes: a short-circuited run capacitor, seized compressor bearings, or excessive liquid refrigerant in the compressor (Lennox notes a hard start kit may be needed) | Technician work: run capacitor, line voltage (contact the utility if voltage at the disconnect is low), wiring connections, and the compressor itself. Clears after a power reset or four normal compressor cycles — do not keep forcing starts against a locked rotor |
| 405 | Compressor open circuit (iComfort alert, outdoor unit) — the compressor circuit is open | Lennox's XC21 cause list: outdoor unit power disconnect open, unit circuit breaker or fuse(s) open, the compressor contactor failed to close (check its wiring, connectors, coil pilot voltage and for burned/pitted contacts), a high-pressure switch open and requiring manual reset, an open circuit in the compressor supply wiring or connections, an unusually long compressor-protector reset time in extreme ambient, or damaged compressor windings | Owner check stops at the disconnect and breaker. The rest is technician work in Lennox's order: contactor, wiring and connectors, winding resistance. Clears after one normal compressor run cycle or a power reset, once the open is actually repaired |
| 406 | Compressor open start circuit (iComfort alert, outdoor unit) — the required amount of current is not passing through the START current transformer | Lennox's XC21 cause list: a failed run capacitor, an open circuit in the compressor start wiring or connections, or a damaged compressor start winding | Technician work: check the wiring and connectors between supply and the compressor S terminal, the run capacitor, and the compressor motor winding resistance. Clears when amperage is detected in the RUN circuit and sensed by the START sensor, or after a power reset |
| 407 | Compressor open run circuit (iComfort alert, outdoor unit) — the required amount of current is not passing through the RUN current transformer | Lennox's XC21 cause list: an open circuit in the compressor start wiring or connections (the table's wording), or a damaged compressor start winding — in the alert text's terms, the run path is not carrying its current | Technician work as for 406: wiring and connectors between supply and the compressor R terminal, winding resistance. Clears when amperage is detected in RUN and sensed by the run/start sensors, or after a power reset, or after 1 normal compressor run cycle |
| 408 | Welded compressor contactor (iComfort alert, outdoor unit; XC21 Table 10 'Welded Contactor') — the compressor always runs / is running continuously | Lennox's listed causes: the compressor contactor has failed to open (welded contacts), or the thermostat demand signal is not connected to the module | Technician work — a compressor that cannot be switched off is a safety fault, not a comfort fault: contactor replacement, demand-signal check. Clears after one normal compressor run cycle or after a power reset following the repair |
| 409 | Secondary low voltage (iComfort alert, furnace/air handler/outdoor unit) — the secondary voltage for the component has fallen below 18 VAC; if it continues for 10 minutes the thermostat will turn the component off | An overloaded control circuit transformer, or low line voltage (contact the utility if voltage at the disconnect is low), or wiring connections dragging the 24 V down (Lennox's listed causes) | Technician low-voltage diagnosis: transformer load, line voltage, wiring connections. Clears after voltage is higher than 20 VAC for 2 seconds, or after a power reset. A control starved of 24 V imitates board failures across the whole system |
| 410 | Low pressure fault (iComfort alert, outdoor unit) — the outdoor unit pressure is below the required limit; the low-pressure switch has opened (XC21 LED form: DS11 green OFF, DS14 red SLOW FLASH). The iComfort guide gives the switch's own numbers: opens at 40 psig (system shuts down) and closes at 90 psig (system restarts) | Lennox's XC21 cause list: restricted airflow over the indoor or outdoor coil, improper refrigerant charge, or an improper metering device installed / incorrect metering device operation; also check that all pressure switches and sensors have secure connections | Remove blockages and restrictions from coils and fans and check both fan motors' current draws first. Charge checking by approach and subcooling and operating pressures against the unit charging charts are certified-technician work. Each opening is a strike — five strikes in one cooling cycle become 411 |
| 411 | Low pressure switch lockout (iComfort alert, outdoor unit) — the low pressure switch has opened 5 times during one cooling cycle, and the thermostat has shut the outdoor unit down (XC21 LED form: DS11 green OFF, DS14 red ON) | The 410 family persisting to five strikes: restricted airflow, wrong charge, or a metering-device fault that was not fixed | Fix the 410 cause first (charge by approach and subcooling per Lennox), then reset by putting the outdoor unit control in test mode or resetting low-voltage power (Lennox's stated reset). A lockout reset without its cause just re-counts to five |
| 412 | High pressure fault (iComfort alert, outdoor unit) — the outdoor unit pressure is above the required limit and the system will shut down (XC21 LED form: DS11 green SLOW FLASH, DS14 red OFF). The iComfort guide's switch numbers: opens at 590 psig (shutdown) and closes at 418 psig (restart) | Excessive head pressure in cooling: a dirty outdoor coil, a failed outdoor fan, overcharge — and in the Alert Guide's fuller checklist, the condenser fan motor, an expansion valve fault, the indoor blower motor, or a clogged refrigerant filter; also incorrect/improper sensor location or connection causing false pressure/temperature readings | Clean the coil, prove the outdoor fan, confirm the outdoor unit is clean and breathing. Charge and pressure verification against the charging charts is certified-technician work. Five strikes in one cooling cycle become 413 |
| 413 | High pressure switch lockout (iComfort alert, outdoor unit) — the high pressure switch has opened 5 times during one cooling cycle, and the thermostat has shut the outdoor unit down (XC21 LED form: DS11 green ON, DS14 red OFF) | The 412 family persisting to five strikes | Per Lennox: check system charge using approach and subcooling temperatures, check outdoor fan operation, and check for dirt or debris blocking airflow to the outdoor unit — then reset by putting the outdoor unit control in test mode or resetting low-voltage power |
| 414 | High discharge line temperature fault (iComfort alert, outdoor unit) — discharge line temperature has exceeded 279°F (137°C); the control de-energises the compressor contactor and the compressor remains off until the temperature drops below 225°F (107°C) (XC21 LED form: DS11 green SLOW FLASH, DS14 red ON) | The compressor running dangerously hot on its discharge side: charge problems or extreme load — Lennox's checking path is operating pressures against the unit charging charts, with a clean coil and unobstructed airflow in and out of the condenser confirmed first | Technician work: pressures and charge verified against the charging chart. Repeated 414s (5 strikes) become the 415 lockout |
| 415 | High discharge line temperature strikes lockout (iComfort alert, outdoor unit) — the discharge-temperature fault count has reached 5 strikes (XC21 LED form: DS11 green FAST FLASH, DS14 red ON) | The 414 condition repeated five times in a single demand | Reset by putting the outdoor board/control in test mode or resetting low-voltage power (Lennox's stated reset) — after the overheating cause is fixed and the charge proven by approach and subcooling |
| 416 | Outdoor coil sensor fault (iComfort alert, outdoor unit — iComfort guide 506920-01 and Alert Code Guide V2 assignment) — the outdoor coil sensor is open, short-circuited or out of sensor range. NOTE: the XC21 manual's own fault table omits 416 and lists only 417 for its sensor fault (see 417) — on an XC21, read a sensor alert with the XC21 table; on iComfort/S30 systems 416 is the coil sensor | A failed outdoor coil temperature sensor, its harness disconnected or in the wrong pin positions, or a sensor reading out of range — the Alert Guide notes the control allows the unit to run 90 seconds before checking sensor resistance, counts one fault per bad reading, and escalates from moderate (Service Soon) to critical after 10 minutes | Technician work: plug the sensor harness in correctly, check the sensor's resistance against the temperature/resistance chart, and replace the sensor if out of specification. Lennox states the control will not perform demand or time/temperature defrost operation while this fault stands (a heat-pump consequence carried in the shared A175 table); the system still heats or cools. Clears when the control detects proper sensor readings |
| 417 | Discharge sensor fault (iComfort alert, outdoor unit) — the discharge-line sensor is open, short-circuited or out of sensor range. On the XC21 this is the ONLY sensor code in its fault table (LED form: DS11 green OFF, DS14 red FAST FLASH) and the iComfort guide words its text generically as 'the outdoor unit sensor'; the Alert Guide and the XP21-family table both name 417 as the discharge sensor specifically | A failed discharge-line sensor, or a sensor loose on its line so its reading is not real, or sensor leads in the wrong pin positions in the harness plug | Per Lennox: the board allows 90 seconds of run before checking resistance and counts one fault per bad reading, locking out after 5 faults (iComfort guide steps). Check the sensor reading and its attachment to the line, compare resistance to the chart, and replace if out of specification. Technician work |
| 418 | W output hardware fault (iComfort alert, outdoor unit) — there is a faulty 'W' output circuit: the W output is energising the W terminal when it should not be energised (in cooling terms, auxiliary-heat output active when it must not be) | A stuck-closed relay on the outdoor control, or something external to the control energising the W terminal (the Alert Guide's diagnostic split) | Per the Alert Guide: disconnect any wiring from the W terminal — if 24 VAC is still present on the terminal it is a stuck relay (replace the outdoor control); if 24 VAC disappears, check the wires hooked to the W terminal. Clears after the fault signal is removed. Technician work |
| 419 | W output fault lockout (iComfort alert, outdoor unit) — the W output has reported more than 5 errors/strikes, and the system has shut the outdoor unit down | The 418 hardware fault reaching its 5-strike limit | Per Lennox: disconnect the thermostat wire from W and verify there is no 24 VAC on W; if 24 VAC is present, replace the outdoor control. Automatically clears after power is recycled — following the repair, not instead of it |
| 421 | W output external miswire fault (iComfort alert, outdoor unit) — the W output terminal on the outdoor unit is not wired correctly: voltage is sensed on the W output terminal when the Y1 output is deactivated | A wiring error at the outdoor unit's W terminal — a mislanded wire putting voltage where the control does not expect it | Installer work: correct the W-terminal wiring at the outdoor unit. Automatically clears once voltage is no longer sensed on the output, or after power is cycled (Lennox's stated clearing) |
How to reset / clear error codes on Lennox air conditioners
Lennox alerts clear by family, and the manuals are explicit about each. The compressor-circuit family (400, 404-408) clears after a stated number of normal compressor run cycles — one cycle for 405 and 408, four for 402/403/404 — or after a power reset, with 400 also clearing once current is sensed in both RUN and START sensors for 2 seconds. The pressure and discharge families clear by threshold: 410 when the switch recloses (90 psig after opening at 40), 412 when the high-pressure switch recloses (418 psig after opening at 590), 414 when discharge temperature falls below 225°F. The lockouts (411, 413, 415, 419) are different: Lennox's stated reset is to put the outdoor unit control in test mode (the FIELD TEST pins, E33, jumpered for 1-2 seconds) or to reset low-voltage power to the control — and that reset is the LAST step of the repair, after charge has been proven by approach and subcooling, because the 5-strike counter simply counts again. Sensor alerts (416, 417) clear themselves when the control again detects proper sensor readings. No Lennox platform here has an owner 'clear all alerts' button; the thermostat's alert screens are for reading and reporting.
How to diagnose any Lennox air conditioner code
Work the problem in this order — and stop where refrigerant, compressors or mains wiring begin, because those are licensed work: (1) record the code exactly as displayed, including whether it came from the indoor display, the wired/wireless remote check mode, or an LED pattern on the outdoor unit — the three are different reports; (2) check the homeowner basics first: the air filter (a matted filter ices coils and imitates sensor and refrigerant faults), the condensate drain and float switch, the outdoor coil clear of leaves and debris, and both breakers fully on; (3) power-cycle once at the breaker, then run the mode the code interrupted and watch the sequence — indoor fan, outdoor fan, compressor start — because the stage that never starts names the fault family; (4) on mini-splits, retrieve the stored check code with the remote's diagnosis function before clearing anything, and write down the full code including its letter family; (5) a code that returns in the same mode is an active fault. Refrigerant charge, sealed-system and board-level work is professional work — an undercharged system re-iced after a reset is being damaged, not cured.
One control, two appliances: why Lennox AC and heat-pump alerts share numbers but not always meanings
Lennox builds its communicating outdoor units around a shared main control — the A175 — and fits it to both air conditioners (XC17, XC21) and heat pumps (XP17, XP21). The shared hardware produces a shared alert vocabulary, and mostly that is a gift: 410 is a low-pressure fault and 411 its lockout on both appliances, 412/413 mirror them on the high side, and 414/415 police the same 279°F discharge limit. But shared is not identical, and the seam shows exactly where a careful reader earns their keep. The XC21 air-conditioner manual's fault table runs 410 through 415 and then lists 417 as its sensor fault — there is no 416 in that table at all. The iComfort thermostat guide and Lennox's later Alert Code Guide, written across the whole communicating range, assign 416 to the outdoor coil sensor and 417 to the discharge sensor. Same number family, same control lineage, two documents, two maps — because the coil sensor's defrost role matters to a heat pump in a way it simply does not to a cooling-only unit, and the tables were written per appliance, not per control. The practical rule follows directly: an alert number is only half a report. '417' quoted with the model — 417 on an XC21, 417 on an XP21 — is a diagnosis a technician can act on; a bare '417' searched on the internet is a coin flip between two sensors at opposite ends of the refrigerant circuit. This page therefore publishes both assignments, scoped to their documents, rather than flattening them into one confident line. When Lennox's own manuals disagree by scope, the correct publication is the disagreement, labelled — anything tidier would be tidier than the truth.
Five strikes: the pressure and temperature lockouts, and why the reset is the last step of the repair
Lennox's A175 control does not lock an air conditioner out on the first sign of trouble, and understanding its patience is the key to its alert table. A pressure switch opening earns a strike and a moderate alert — 410 on the low side, 412 on the high side — the compressor stops, the switch recloses when pressure recovers (the iComfort guide publishes the switch's own arithmetic: low side opens at 40 psig and recloses at 90, high side opens at 590 and recloses at 418), and the system tries again. Only the fifth strike inside a single cooling cycle converts the alert into a lockout — 411, 413 — and shuts the outdoor unit down until a human intervenes. The discharge-temperature family runs the same logic on a thermometer instead of a switch: above 279°F is alert 414, the compressor held off until the line cools below 225°F, and five strikes make lockout 415. Even the sensor and W-output faults count to five before locking out. This design is a diagnostic gift if you read it whole. A unit showing 410 has a pressure problem that has happened once; a unit showing 411 has the same problem five times in one cycle, which rules out the freak gust and the one-off glitch and points squarely at the short cause list Lennox prints: airflow, charge, metering device. The reset Lennox prescribes — test mode via the FIELD TEST pins, or cycling low-voltage power — is deliberately placed at the END of its own instructions, after 'check system charge using approach and subcooling temperatures'. Performing it first does not save the diagnostic fee; it spends the strike history, restarts a compressor into the condition that just stopped it five times, and guarantees the lockout returns — with the evidence erased. Fix, prove, then reset, and the code stays gone because its cause did.
Reading a Lennox that cannot speak: non-communicating units, the A177 fan control, and the alerts hiding behind the installer button
Not every Lennox air conditioner can tell you what is wrong with it, and part of reading this brand well is knowing which units are mute. The numbered alert table on this page belongs to the communicating generation — A175-controlled outdoor units paired with an iComfort or later S-series thermostat. The long-selling non-communicating Merit and Elite units, wired to conventional 24 V thermostats, publish no alert table in their manuals because they carry no reporter: no communicating control, no thermostat alert screen, nothing to read. For those units, diagnosis remains the older craft of pressures, temperatures and current draws, and any website offering you a 'code list' for one is lending it the communicating fleet's voice. Two quieter reporters deserve attention even on communicating systems. The first is the A177 fan-motor control on XC21-class units, which speaks its own miniature language through a single LED: a fast flash with no pause means the fan's PWM feedback does not match its target speed, a constant-on LED means the control's own CRC check has failed, and both end at the same repair — replace the fan motor control — that no thermostat alert will ever announce as plainly. The second is the priority system itself. Lennox shows critical alerts on the homeowner's home screen, but moderate alerts — the first 410, the first 412, the early warnings that precede every lockout — live only behind the installer alert button. A homeowner can therefore honestly report 'no alert, it just stopped cooling' about a system whose installer screen holds a complete diary of strikes. Always read the installer list before concluding anything from an empty home screen: on this platform, silence at the surface and a full history underneath is the normal state of a fault caught late.
Less common and model-specific codes
Search data shows owners also meet these codes on Lennox air conditioners: 422, 423, 424. They are not part of the core code list Lennox publishes for most models — several are series-specific, abbreviated display readings, or codes carried over from a sister platform — so we do not publish meanings for them rather than guess. Treat them by symptom: note what the machine was doing when the code appeared (filling, draining, heating, spinning), run one full power cycle (5 minutes off), and if the code returns, match the symptom to the closest documented family in the table above and check the tech sheet stored inside your machine for the model-specific definition.
Related guides
Facts on this page were checked against manufacturer documentation: cdn3.servls.com · manualmachine.com · www.delairteam.com · www.complete.impossiblewebdesign.com