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Heat Pump Error Codes: Every Brand, Every Code Explained

All brands Heat Pump display showing ALL CODES — FixCode Guide error code graphic

A heat pump's error code is never just a number — it is a message from a machine that lives in two worlds at once. In summer it is an air conditioner; in winter it reverses its refrigerant flow and pulls heat out of cold outdoor air, defrosting its own coil every so often and calling on auxiliary heat when the weather wins. Each manufacturer reports trouble from a different chair in that machine: Carrier and Rheem show two-digit codes on the outdoor control, Trane counts flashes on its defrost board, Goodman narrates on a 7-segment display, and Lennox speaks in pairs of LEDs or in thermostat alerts. This hub gathers the verified tables for all five brands, explains how to read each one's language, and — just as important — separates the displays that describe normal winter behaviour from the codes that describe a machine asking for help.

Safety first. Every heat pump combines a sealed refrigerant circuit (high-pressure side switch settings in these manuals run to 590 psig), mains electricity (208–240 V, plus control transformers and, on inverter models, drive boards that hold charge after isolation), a compressor with a start circuit, and an outdoor fan that can start without warning when a control retries a demand. Codes are read from outside the cabinet, at the thermostat, or from the displays the manufacturers intend to be read — never by opening the unit, jumpering a pressure switch, or holding a contactor in. Pressure switches and lockouts are compressor protection, not bureaucracy: defeating them converts a repairable fault into a destroyed compressor. Refrigerant charging, leak repair, pressure-switch testing and inverter work belong to a certified technician; your job is the exact code, the brand, and what the unit was doing when it appeared.

How heat pumps report faults: five brands, five languages

Heat pump faults are reported from the outdoor unit, because that is where the compressor, the pressure switches and the defrost logic live — but each brand chose a different display. Carrier's communicating outdoor control flashes a two-digit code on an amber LED (short flashes for the first digit, long for the second), and the same fault appears in words at the Infinity wall control. Trane's electronic demand defrost control counts single flashes on a board LED — one to nine flashes, repeating — while its healthy status light simply blinks once a second at idle and twice a second on a call, so on a Trane, rhythm itself is information. Goodman's communicating control uses a two-character 7-segment display that alternates between characters (01, A2, L1), and mirrors the event at a ComfortNet thermostat as a plain-language message. Lennox's defrost control uses no digits at all: two LEDs, green and red, combine — both flashing slowly together means normal operation, and the fault patterns are the other combinations, with solid lights marking lockouts; Lennox's communicating models translate the same events into numbered alerts at the iComfort control. Rheem's Integrated Compressor Control shows letters for operating states (0 standby, h/H heating stages, d defrost) and two digits for faults on a dual 7-segment display, with EcoNet systems adding A/T alert numbers at the wall.

Two reading rules survive every brand. First, identify the control generation before the code: a number that means 'high pressure control open' on one generation may not exist on the next, several brands publish entirely different tables for communicating and non-communicating models, and some older models publish no fault codes at all — a silent timed defrost board is a design, not a failure to communicate. Second, distinguish trips from lockouts. Across all five brands the grammar is the same: a single event is a report (often self-clearing), repetition inside one demand is an escalation, and the lockout form — Carrier's 83/84, Trane's 4-flash hard lockout, Goodman's L1/L2, Lennox's solid LEDs at five strikes, Rheem's flashing L-codes — is the control announcing that the fault is a condition, not a moment. Quote the code with its form: 'tripped once' and 'locked out' are different service calls.

Defrost is usually not a fault

The most-reported heat pump 'fault' every winter is the defrost cycle, and it is not a fault at all. To heat your home, the outdoor coil must run colder than the outdoor air, so in cool, damp weather frost grows on it; the unit periodically reverses itself into cooling, stops the outdoor fan, melts the frost with hot refrigerant, and energises auxiliary heat indoors so the vents do not blow cold. The visible drama — steam rising off the unit, a whoosh as the reversing valve shifts, the outdoor fan stopped while the compressor runs, cooler air at the vents for a few minutes — is the cycle working exactly as the manuals describe. Brands even give it display space: Rheem shows 'd', Goodman shows 'dF', and Lennox's homeowner literature warns in advance that frost below about 45°F, defrosts of 5 to 15 minutes, steam, and auxiliary heat during defrost are all normal.

The line between normal and fault is pattern, not presence. A healthy defrost happens when the coil needs it (Trane's demand control targets about four minutes and caps at fifteen; Rheem's algorithm requires at least 34 minutes of cold-coil runtime before initiating and terminates at a 60°F coil temperature or 14 minutes), ends with a clear coil, and leaves long stretches of ordinary heating between events. A defrost problem looks like defrost constantly, a coil that never clears, or ice that grows day over day — and those states surface as sensor and pressure codes, not as a 'defrost error', because most brands publish no defrost fault code at all. The sensors that steer defrost (ambient and coil thermistors) are therefore the quiet centre of winter reliability: Carrier degrades to timed defrost with five-minute terminations when they fail, Trane falls back to scheduled 60-minute defrosts, and Lennox's coil-sensor failure stops defrost entirely. If your unit ices up, the first evidence to gather is which sensor pattern its board is showing — before anyone suspects the refrigerant.

The heat-pump sequence: reversing valve, outdoor fan, compressor, auxiliary heat

Reading any heat pump code starts with the sequence it interrupted. In heating, the indoor thermostat's call starts the reversing valve's job first: the valve positions the refrigerant flow so the outdoor coil becomes the evaporator. The compressor then pumps hot, high-pressure refrigerant to the indoor coil, where it gives up its heat to indoor air; the outdoor fan pulls cold air across the outdoor coil so the refrigerant can absorb what heat the winter air holds; and the whole cycle leans on the indoor blower moving enough air to carry the heat away. Auxiliary heat — electric strips or a gas furnace — waits behind the heat pump as second and third stages, engaged when the heat pump alone cannot hold the setpoint, when defrost needs tempering air, or when a lockout has benched the heat pump entirely. Emergency heat is the manual version of that last state: auxiliary heat only, heat pump locked out by the homeowner's own thermostat setting.

Every family's codes map onto that chain. Pressure codes (Carrier 31/32, Goodman 01/02, Rheem 21/29, Lennox's flash patterns, Trane's 3-flash) are the chain's load limits talking: low pressure means the outdoor coil is starved (low charge, ice, a stopped fan, a closed expansion device), high pressure means heat is not being carried away (dirty coil, failed fan, weak indoor airflow). Sensor codes (Trane 1/2 flash, Carrier 53/55, Goodman A2/A3, Lennox's fast-flash pairs, Rheem 83/84) are the chain's instruments failing. Compressor-circuit codes (Rheem 06/07 and lockouts, Carrier 71–74) are the chain's engine failing to turn. And the defrost displays are the chain deliberately running backwards for a few minutes. Ask of any code, 'which link is speaking?' and the repair conversation shortens immediately — an airflow problem and a refrigerant problem can produce the same pressure code, but they are different trades of work at different prices, and the manuals' cause lists, quoted accurately, separate them.

Carrier heat pump error codes

Carrier's heat-pump fault table belongs to its communicating outdoor control (Infinity family): two-digit codes flashed on an amber LED and mirrored at the Infinity wall control. The shape of the table is pressure-centred — 31 high pressure switch open, 32 low pressure switch (or, on this control, discharge-temperature switch) open — with a retry system that attempts the compressor every 15 minutes and lockouts (83/84) only after three consecutive tripping cycles. Around that core sit the electrical reports (47 no 230 V at unit, 74 no 230 V at compressor, 73 contactor shorted, 46 brownout) and the thermistor family (53/55/56) that quietly governs defrost fallback. Carrier's non-communicating heat pumps use timed defrost boards with no published code table — if the unit has no Infinity control, this hub's Carrier table does not apply to it.

Code	Meaning
1 flash, pause	Operation indication, not a fault: low-stage cooling or heating is running (shown on the control's operation LED)
2 flashes, pause	Operation indication, not a fault: high-stage cooling or heating is running
16	System communications failure — communication with the user interface (wall control) has been lost
25	Invalid model plug — the control does not detect a model plug, or detects an invalid one; the unit will not operate without the correct model plug
31	High pressure switch open — a high-pressure switch trip has stopped the compressor (on this control the code carries Carrier's retry sequence: contactor off, outdoor fan runs up to 15 minutes, restart attempted every 15 minutes)
32	Low pressure switch open, or discharge temperature switch open — a low-pressure or discharge-temperature trip has stopped the compressor (same 15-minute retry sequence as code 31)

Full list: Carrier Heat Pump Error Codes List — every verified Carrier code with causes and first fixes.

Trane heat pump error codes

Trane's split heat pumps report from the electronic demand defrost control in the outdoor unit: a counted flash code, 1 through 9, on the board's Fault LED. The table is small and mostly about winter: sensor failures that change the defrost fallback (1 and 2 flashes; 7 and 8 on the earlier board generation), the low-pressure ladder (3 flashes trip, escalating soft lockouts at 5 flashes, hard lockout at the ninth trip shown as 4 flashes), and a low-ambient lockout (9 flashes) with generation-specific floors of −7°F or −12°F. Because the same board also blinks a healthy status rhythm, Trane diagnosis begins with rhythm: one flash a second idle, two on a call, anything else a fault. Trane's numbered ComfortLink communicating alerts are a separate system published in a separate addendum and are not this table.

Code	Meaning
1 FLASH	Ambient temperature sensor out of range (open or shorted) — the defrost control falls back to initiating a defrost after every 60 minutes of runtime, terminating on coil temperature
1 flash / second	Status, not a fault: the defrost control's status LED flashes once each second when idle, and twice each second with a call for heating or cooling — that steady rhythm is Trane's documented sign of normal operation
2 FLASH	Coil temperature sensor out of range (open or shorted) — the control initiates a 15-minute forced defrost after every 60 minutes of runtime
3 FLASH	Low pressure switch (LPCO) is open — the low-pressure cut-out has tripped; the 3-flash indication goes away when/if the LPCO closes again
4 FLASH	Hard lockout — the compressor is locked out and system operation is interrupted; it can only be cleared with a (low-voltage) power cycle. It occurs after the 9th trip of the LPCO
5 FLASH	Soft lockout — the heat pump is paused for a timed lockout period (15 minutes on the first trip, escalating 30, then 45 minutes, then an 18-hour period on later trips) and the 5-flash indication goes away when the period expires

Full list: Trane Heat Pump Error Codes List — every verified Trane code with causes and first fixes.

Goodman heat pump error codes

Goodman's verified heat-pump codes come from its communicating outdoor control (PCBHR104/105) on ComfortNet systems: two-character codes on a 7-segment display, mirrored as plain-language messages at the thermostat. The table mixes true faults (01/02 pressure trips, L1/L2 lockouts, 06/07 start/run circuits, E5 fuse, EE control) with heat-pump-only status displays that owners routinely misread as faults (dF defrost, dt maximum defrost time, dE forced defrost). The larger Goodman fleet — the non-communicating GSZ generation — defrosts on a time/temperature board (checks every 30/60/90 minutes, 10-minute maximum defrost) and publishes no codes at all, so the most common Goodman answer is 'your model does not report codes; here is what its sequence should be doing.'

Code	Meaning
01	Low pressure cut-out trip / low side fault — the low pressure switch has opened (ComfortNet thermostat message: LOW SIDE FAULT / LPS OPEN)
02	High pressure cut-out trip / high side fault — the high pressure switch has opened (also the code shown after four consecutive compressor protector trips with average run time between 1 and 15 minutes, with both pressure switches closed)
04	Locked rotor — the control has detected a compressor locked-rotor condition
06	Open start circuit — current is present in the run circuit but not in the start circuit
07	Open run circuit — current is present in the start circuit but not in the run circuit
08	No line voltage — the control detects no line voltage at the unit

Full list: Goodman Heat Pump Error Codes List — every verified Goodman code with causes and first fixes.

Lennox heat pump error codes

Lennox publishes two heat-pump languages. Merit/Elite models read from the demand defrost control's two LEDs: simultaneous slow flash is normal, alternating slow is the five-minute anti-short-cycle delay, the fast-flash pairs are the ambient and coil sensors, and the four green/red pairings split pressure faults (flashing) from five-strike lockouts (solid). The control's strike counter and its documented blind windows (the low-pressure switch is ignored during defrost, for 90 seconds after start-up, and for 90 seconds after the reversing valve changes mode) make pattern timing as diagnostic as the LEDs themselves. Communicating Signature models (XP17/XP21 with iComfort) add numbered outdoor alerts — 180 for the ambient sensor and the 413–419 group for discharge-temperature, pressure-lockout and W-output faults — with one documented trap: alert 417 means the coil sensor on the XP17 family and the discharge sensor on the XP21 family, so the model decides the meaning.

Code	Meaning
180	Ambient sensor problem (iComfort alert, communicating models) — the outdoor ambient sensor is open, shorted or out of range; the heat pump control reverts to time/temperature defrost and the system still heats or cools
413	High pressure switch lockout (iComfort alert, XP21-family outdoor unit) — the high pressure switch has locked the unit out
414	High discharge line temperature fault (iComfort alert, XP21-family) — discharge line temperature has exceeded 279°F; the alert clears when it falls below 225°F
415	High discharge line temperature strikes lockout (iComfort alert, XP21-family) — the discharge-temperature fault count has reached 5 strikes
416	Outdoor coil sensor fault (iComfort alert, XP21-family) — the sensor is detected open, shorted or out of range, and the outdoor unit control will not perform demand or time/temperature defrost operation
417	Discharge sensor fault (iComfort alert, XP21-family) — the discharge sensor is detected open, shorted or out of range; the board allows 90 seconds of run before checking resistance, counts one fault per bad reading, and locks out after 5 faults. (On the earlier XP17-family control the number 417 is instead attached to the coil-sensor problem — see the scope note in this file's header.)

Full list: Lennox Heat Pump Error Codes List — every verified Lennox code with causes and first fixes.

Rheem heat pump error codes

Rheem's heat pumps report through the Integrated Compressor Control's dual 7-segment display — the same display language as Rheem's communicating furnaces but a different, heat-pump vocabulary: operating letters (0, c/C, h/H, d) plus two-digit faults and alternating L-code lockouts that Rheem's manuals require to be manually reset. Defrost on these models is an algorithm inside the ICC rather than a separate board, with documented initiation conditions (coil below 35°F, 34 minutes of cold-coil runtime) and termination at 60°F or 14 minutes. The pressure pair 21/29 carries full mode-specific cause lists (a 29 in heating points indoors — filter, coil, blower — as readily as a 29 in cooling points outdoors), and EcoNet variable-speed models pair each display code with an A/T alert number at the wall control. Rheem furnace codes do not transfer to this table, and vice versa.

Code	Meaning
0	Standby — not a fault: the ICC display reads 0 when there is no command for unit operation
01	Long run time (two-stage ICC list) — the compressor has run continuously for more than 18 hours in cooling mode
02	High pressure (two-stage ICC list) — the compressor limit has opened four times within a call
03	Short cycling (two-stage ICC list) — the run time of each of the past four cycles was less than three minutes (on the variable-speed manual, repeated short runs surface as the '3' fault, reset by removing low-voltage power from the outdoor control board)
05	Open circuit (two-stage ICC list) — the ICC has had a protector trip lasting longer than 4 hours
06	Compressor open start circuit (two-stage ICC list) — current is measured in the run circuit but not in the start circuit

Full list: Rheem Heat Pump Error Codes List — every verified Rheem code with causes and first fixes.

When a heat-pump code is a pro-only job

Some codes can be triaged from the sofa — a status display, a defrost in progress, a first pressure trip after a visibly dirty filter — but the manuals draw a consistent line beyond which the work is a certified technician's, and it is worth respecting because the hazards compound. Anything involving refrigerant is pro-only by law and by physics: low-pressure codes that repeat (the charge, leak or expansion device behind them), restrictions, and any pressure-switch testing. Anything involving the compressor's own circuits is pro-only by consequence: locked-rotor and start/run lockouts (Rheem L4/L6/L7, Carrier 71–74) mean the next start attempt is the damaging one, and resetting first converts diagnosis into destruction. Inverter and variable-speed faults add stored electrical energy to the list; Rheem's own instruction is to remove power until every light on the drive has gone out before touching anything. And any lockout — the L-codes, the solid LEDs, the hard 4-flash — is the control formally requesting a human decision: its cause repaired first, its reset second. The professional visit goes better with evidence: the exact code and its form (trip or lockout), the brand and control generation, whether the house was heating on auxiliary heat, and what the weather was doing. That is the difference between a diagnosis and a parts-changing expedition.

Seasonal patterns: the first cold snap, deep winter, and the shoulder seasons

Heat pump codes arrive on a calendar. The first cold snap of autumn exposes everything the cooling season hid: systems that leaked charge all summer announce it as low-pressure trips on the first long heating runs (Goodman 01, Rheem 21, Trane's climbing trip count), marginal defrost sensors fail their first real test, and units that spent months idle reveal start-circuit faults. Because auxiliary heat silently covers for a benched heat pump, the first cold snap's other signature is financial — a normal-feeling house with an abnormal bill — and the code check should happen when the bill surprises, not when the house finally goes cold. Deep winter is defrost season: demand-defrost systems (Trane, Lennox, Rheem's ICC) cycle as often as their algorithms allow, timed systems (Goodman's board, sensor-failed fallbacks everywhere) cycle on their fixed intervals, and the faults that surface are the defrost-adjacent ones — coil and ambient sensor codes, pressure trips from iced coils, and the low-ambient lockouts that are not faults at all (Trane's documented floors at −7°F or −12°F by generation). A unit that locks out only on the coldest nights and recovers by midday may be obeying its manual, and the correct purchase is auxiliary-heat capacity or a balance-point adjustment, not a compressor.

The shoulder seasons write the subtler entries. In spring, heating-side sensor faults that degraded defrost all winter go unnoticed as defrost demand fades — until the same thermistors misreport for cooling decisions, which is why a winter sensor code deserves a spring repair rather than a spring shrug. Mild-weather short-cycling codes (Rheem 03, Goodman's 03-style displays) appear when an oversized system meets a small load, and brownout codes (Carrier 46) cluster on the first hot afternoons when a whole street's compressors start together. The calendar's lesson is that a code's meaning includes its month: technicians read the same two digits differently in January and June, and the hub tables' cause lists — which separate heating-mode from cooling-mode causes for exactly this reason — reward owners who report the season along with the code.

How to diagnose any 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.

Frequently asked questions

My heat pump is steaming and the outdoor fan stopped. Is it broken?

Almost certainly not — that is a defrost cycle. The unit has briefly reversed into cooling to melt frost from the outdoor coil: the fan stops, steam rises off the melting frost, the reversing valve whooshes, and auxiliary heat tempers the indoor air. Documented defrosts run roughly 4–15 minutes depending on brand and conditions, and the coil should be clear afterwards. It becomes a problem only when the pattern changes: defrosting near-constantly, a coil that never clears, or ice that grows for days — those point to the sensor and pressure codes in the brand tables above.

The display shows a defrost code (d, dF, dE, dt). What do I do?

Nothing — those are status displays, not faults. Rheem's ICC shows 'd' while defrosting; Goodman's communicating control shows dF during defrost, dE for a forced defrost, and dt when a defrost ended at its maximum-time limit rather than on temperature. A single dt is unremarkable; dt on every cycle, or defrosts so frequent the house cannot hold temperature, is worth a service call with the observation noted.

Why is my house warm but my electricity bill doubled after a code appeared?

Because a locked-out heat pump does not stop the heating — the thermostat silently moves the load to auxiliary electric heat, which costs several times more per unit of warmth. Trane's soft lockouts even pass the call to auxiliary heat by design while the heat pump sits out its penalty. A warm house plus a shocking bill in cold weather is a classic lockout signature (Carrier 83/84, Goodman L1/L2, Rheem L21/L29, Lennox solid LEDs): read the outdoor unit's display before assuming the meter is wrong.

Can I just reset a lockout myself to get the heat pump back?

You can — every brand's lockout clears with a power cycle or a documented reset — but the manuals are unanimous that the reset is the last step, not the first. A lockout means the same fault repeated inside one demand (three pressure trips on Carrier/Goodman/Rheem, five strikes on Lennox, nine trips on Trane's hard lockout). Resetting without repairing buys one more run into the same damaging condition, and on compressor-circuit lockouts (locked rotor, open start/run) the next start attempt is itself the injury. Fix the listed cause, then reset.

My heat pump stopped working on the coldest night but was fine the next morning. Was it a fault?

Possibly not. Some controls have documented low-ambient floors: Trane's defrost control locks the heat pump out at −7°F (earlier generation) or −12°F (later generation) and resumes a few degrees warmer, handing the house to auxiliary heat meanwhile. If the 'failure' tracks extreme cold exactly and recovers with the temperature, check your brand's floor in its hub table before paying for a diagnosis — and if it happens often, the fix is auxiliary-heat capacity or balance-point settings, not parts.

The same code number means different things in different lists I found online. Which is right?

The one for your control generation. Heat-pump reporting changed repeatedly: Rheem's ICC shares display habits with its furnaces but not their fault list; Lennox's alert 417 is the coil sensor on one communicating family and the discharge sensor on another; Trane's flash table belongs to its defrost board, not its ComfortLink thermostat alerts. Generic cross-brand code lists blend these freely and are the main source of wrong diagnoses. Identify your unit's control (communicating display, defrost-board LEDs, or none) from your brand hub first, then read the code in that table.

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