Lennox Heat Pump Error Codes List: Every Code Explained

Every Lennox heat pump 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 heat pumps

The defrost control speaks through two LEDs — green DS2 and red DS1 — and the combination is the code: both flashing slowly together is normal operation, alternating slow is the anti-short-cycle delay, simultaneous fast is the ambient sensor, alternating fast is the coil sensor, both solid is a failed control, and the four green/red on-off pairings separate low- and high-pressure faults from their lockouts. There is nothing to count and no digits to read on these models — owners misreport Lennox more than any other brand here because a gentle simultaneous slow flash (a healthy standby) gets described as 'flashing a code'. Communicating models repeat the outdoor story at the iComfort thermostat as numbered alerts (180 ambient sensor; the 413–419 outdoor group), which is the form to quote when the system has a wall control.

The complete Lennox heat pump error code list

How to reset / clear error codes on Lennox heat pumps

On the defrost-control models, pressure faults strike-count: each opening of a pressure switch adds one strike to that code's counter, and five strikes in a single demand cycle become a lockout (red LED solid for low pressure, green solid for high). The unit stays locked out until power to the defrost control is interrupted and re-established, or a jumper is applied to the TEST pins for 0.5 seconds. The anti-short-cycle delay (alternating slow flash) needs no reset at all — it expires in five minutes. On communicating models, Lennox gives alert-specific resets: the 413/415 lockouts reset by putting the outdoor control in test mode or resetting low-voltage power, 414 clears itself when discharge temperature falls below 225°F, and sensor alerts clear when the control again detects proper readings.

How to diagnose any Lennox heat pump code

Work the problem in this order — and stop where the refrigerant circuit, the compressor or mains wiring begins, because those belong to a licensed HVAC technician with gauges: (1) record the code exactly, and note where it appeared — the outdoor board's LED, a communicating wall control, or an app — because one heat pump can report the same fault three different ways; (2) run the free checks first: the air filter is clean, the indoor coil is not iced over, the outdoor unit is clear of leaves, snow and debris, both breakers are on, and the condensate drain is not lifting a float switch; (3) remember that defrost is not a fault — in cold weather the outdoor fan stops, the unit steams and hisses and the indoor air cools for a few minutes while frost melts off the outdoor coil; a defrost that never ends, or ice that never clears, is the fault, not the defrost itself; (4) clear the code once and watch a full heating cycle from the start — the stage that stops (outdoor fan, compressor, defrost, auxiliary heat) names the fault family; (5) pressure lockouts, reversing-valve faults and compressor faults are diagnosed with gauges and a meter on the sealed circuit — never "top up" refrigerant to make a code go away, because a heat pump losing charge has a leak, and the code is protecting the compressor from it.

The strike counter: how Lennox's defrost control counts to five, and the three windows where it refuses to count

Lennox's demand defrost control does not trip; it counts. Both its pressure switch circuits — the high pressure switch (S4), factory-wired to the control's HI PS terminals, and the low pressure (loss-of-charge) switch (S87) — feed a strike counter: each fault adds one strike to that code's counter, and five strikes become a lockout, shown as a solid LED on the fault's side (red solid for low-pressure lockout, green solid for high-pressure lockout), against the slow-flash pattern of a mere fault. The lockout endures until power to the defrost control is interrupted and re-established, or a jumper touches the TEST pins for half a second. The numbers behind the switches are worth knowing: on the ML17XP1, the high pressure switch opens when liquid-line pressure rises above 590 ± 15 psig and resets itself at 418 ± 15 psig, while the low pressure switch opens below 25 ± 5 psig on the suction line and closes again at 40 ± 5 psig. Equally important are the three windows in which the control deliberately ignores the low-pressure switch: during TEST mode, during the defrost cycle itself, and during the 90 seconds after start-up — plus the first 90 seconds each time the reversing valve switches between heat and cool. Those windows exist because pressure dips legitimately during defrost and changeover, and a control that counted them would lock out healthy units every winter morning. So a Lennox low-pressure fault pattern that appears only in the minute after a defrost ends sits in a grey zone the manual anticipates — the control ignored the switch then, so a pattern showing at that moment is describing something else, or a dip long enough to outlast the window. The homeowner's translation of all this: a flashing asymmetry (green off, red slow-flashing, or green slow-flashing, red off) is a strike being recorded, not a shutdown, and the unit may run for weeks accumulating them in single demand cycles that keep ending before five. The day it reaches five inside one cycle, the LED goes solid and the house goes onto auxiliary heat. Reporting 'it flashed the low-pressure pattern for a month, then locked out' gives a technician the whole story — a slow leak or a starving airflow path — before a single gauge is connected.

Two sensors, two very different failures — and Lennox's rules for frost, steam and the six-hour warm-up

Lennox's defrost control trusts two temperature sensors, and its manuals assign their failures opposite severities. If the ambient sensor is detected open, shorted or out of range (simultaneous fast flash on the defrost control; alert 180 on communicating models), the control reverts to time/temperature defrost operation and the system still heats or cools — degraded, but fully defrosting. If the coil sensor fails (alternating fast flash; the condition the XP17-family control numbers as alert 417), the manual's sentence is blunt: the heat pump control will not perform defrost operations. That asymmetry is the single most useful Lennox fact in this hub. An ambient-sensor fault costs efficiency; a coil-sensor fault costs the coil, because in freezing weather a heat pump that cannot defrost ices progressively until airflow through the outdoor coil stops and the pressure faults begin striking. A Lennox that ices up should therefore have its LED pair read before its refrigerant is suspected. Around those faults, Lennox's homeowner documentation sets normal expectations that prevent false alarms: frost develops on the outdoor coil during the heating cycle whenever temperatures fall below about 45°F, an electronic control activates a defrost lasting 5 to 15 minutes at intervals to clear it, steam rising from the outdoor unit during defrost is normal, and the thermostat may engage auxiliary heat during the cycle. In other words, a steaming, hissing Lennox on a cold morning with temporarily cooler vent air is a photograph of health, not of failure — the failure photographs are the ones where the ice never leaves. Finally, Lennox publishes a rule few owners know until it costs a compressor: the crankcase heater. After an extended power outage, the manual instructs setting the thermostat to EMERGENCY HEAT — in which all heating demand is satisfied by auxiliary heat and the heat pump is locked out — for a six-hour compressor crankcase warm-up before switching back to HEAT. Refrigerant migrates into cold compressor oil during a long outage, and starting into that mixture is how compressors die in the first cold snap after a storm. The rule costs one evening of auxiliary heat; the exception costs a compressor. On the communicating Signature models, the same philosophy appears as numbers rather than LED pairs, and the outdoor group is worth knowing as a set: 413 is the high-pressure switch lockout, reset by test mode or a low-voltage power reset; 414 reports high discharge-line temperature, tripping above 279°F and clearing itself when the temperature falls below 225°F; 415 is that temperature fault's five-strike lockout; and 418/419 cover the W (auxiliary heat) output itself — a hardware fault announced by three fast flashes and a pause on the outdoor control, escalating to a five-error lockout that shuts the outdoor unit down. The trap inside the set is 417: on the XP17 family it is the coil-sensor fault (the no-defrost condition above), while the XP21 family's documents assign 417 to the discharge-temperature sensor and give the coil sensor 416 — identical digits, different sensors, different urgency. It is the clearest illustration in this hub of why Lennox codes are always quoted with the model attached: '417' alone is not a diagnosis, but '417 on an XP21' is.

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Facts on this page were checked against manufacturer documentation: manuals.plus · homeappliance.manualsonline.com · manualsdump.com · www.delairteam.com