Carrier Heat Pump Error Codes List: Every Code Explained
Every Carrier heat pump error code we could verify against Carrier'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 Carrier heat pumps
The outdoor control reports through an amber STATUS LED in the outdoor unit: a two-digit code flashed as short flashes (0.25 seconds) for the first digit and long flashes (1.0 second) for the second, with 2.5 seconds off between repetitions — Carrier's own example is 3 short + 2 long = 32, low pressure switch open. A separate operation LED shows running states (1 flash low stage, 2 flashes high stage; 5 or 6 flashes merely report the user's brownout-protection selection, a setting and not a fault). On Infinity systems the same faults are presented at the wall control, which is where most owners first see them. There is no display on the outdoor unit itself; the flash code is read with the control-box cover in place through the sight arrangement the manual describes, not by propping the cabinet open.
The complete Carrier heat pump error code list
| Code | Meaning | Most likely cause | First fix |
|---|---|---|---|
| 1 flash, pause | Operation indication, not a fault: low-stage cooling or heating is running (shown on the control's operation LED) | Normal operation — the unit is answering a low-stage demand | No repair needed. Faults are reported separately as two-digit codes on the amber status LED |
| 2 flashes, pause | Operation indication, not a fault: high-stage cooling or heating is running | Normal operation — the unit is answering a high-stage demand | No repair needed |
| 16 | System communications failure — communication with the user interface (wall control) has been lost | A break, short or miswiring in the ABCD communicating wiring between the user interface, the indoor unit and the outdoor unit | Correcting the wiring fault clears the code and re-establishes communication (Carrier 50CR manual). Communicating-bus wiring diagnosis is technician work; do not swap ABCD wires by trial |
| 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 | Model plug missing, loose, or the wrong plug fitted after a control change | A technician fits the correct model plug for the exact model — the plug carries the unit's identity and capacity data |
| 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) | High refrigerant pressure: check refrigerant charge, outdoor fan operation, and the coils for airflow restrictions (Carrier's stated causes) | Clear coil/airflow restrictions and confirm the outdoor fan runs; charge and pressure-switch work is certified-technician work. Repeated trips escalate to code 84 lockout |
| 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) | Low refrigerant charge or restricted indoor airflow are Carrier's stated checks; in heating the same trip family covers a frosted outdoor coil starving the circuit | Check the indoor filter and airflow first; charge measurement, leak repair and switch testing are certified-technician work. Repeated trips escalate to code 83 lockout. (The 25HNB6 manual's own worked example — 3 short flashes, 2 long — is this code.) |
| 45 | Control fault — the outdoor unit control board has failed | Internal failure of the outdoor control board | Control board replacement is technician work; Carrier's table gives no field repair for 45 |
| 46 | Brown out (230 V) — line voltage fell below 187 V for at least 4 seconds; compressor and fan operation is not allowed until voltage rises above 190 V (the 50CR manual describes the same event on the transformer secondary: below 15 VAC for over 4 seconds, clearing above 17 VAC) | A sagging supply: utility brownout, a failing disconnect/contactor, or undersized/damaged supply wiring dropping voltage under load | Verify line voltage at the unit. A one-off utility sag clears itself; a recurring 46 is electrician/technician work — do not keep restarting the compressor into low voltage |
| 47 | No 230 V at unit — there is no 230 V at the contactor while the indoor unit is powered and a cooling/heating demand exists | Outdoor disconnect open or pulled, a tripped breaker/fuse, or broken 230 V wiring to the outdoor unit | Verify the disconnect is closed and 230 V wiring is connected (Carrier's stated action). Anything beyond the disconnect check is electrician/technician work |
| 53 | Outdoor air temperature (OAT) sensor fault — the sensor is not reading or is out of range | An open or shorted thermistor, or damaged sensor wiring (the 50CR manual: check the wiring to the OAT terminals and ohm the thermistor) | A technician ohms out the sensor and repairs wiring or replaces the sensor; correcting the fault clears the code. Note the documented side effect: with OAT failed, low-ambient cooling is not allowed and defrost is initiated on coil temperature and time instead |
| 55 | Outdoor coil temperature (OCT) sensor fault — the coil sensor is not reading or is out of range | An open or shorted coil thermistor or its wiring, or a sensor no longer secured tight on the coil stub tube | A technician ohms out and reseats or replaces the sensor. Documented side effect of an OCT failure: defrost still occurs at each time interval in heating, but terminates after 5 minutes |
| 56 | Thermistors out of range — an improper relationship between the coil sensor and the outdoor air sensor readings | One of the two thermistors is drifting or misplaced, so the two readings no longer agree with each other | A technician ohms out both sensors and checks wiring and placement. With a thermistor-out-of-range error, defrost runs at each time interval and terminates after 5 minutes (25HNB6 manual) |
| 71 | Low-stage thermal cutout — compressor operation was detected, then disappeared while low-stage demand still existed | An internal compressor overload trip, or a start relay/capacitor held in circuit too long (if installed) | Compressor overload and start-component diagnosis is technician work. Three consecutive cycles with a thermal cutout escalate to the code 81 lockout |
| 72 | High-stage thermal cutout — compressor operation was detected, then disappeared while high-stage demand still existed | An internal compressor overload trip, or a start relay/capacitor held in circuit too long (if installed) | Technician work, as for code 71. Three consecutive cycles escalate to the code 82 lockout |
| 73 | Contactor shorted — compressor voltage is sensed when there is no demand for compressor operation | A contactor stuck (welded) closed, or a wiring error feeding the compressor | A compressor that can be powered with no call is a safety problem: switch the unit off at the disconnect and call a technician — contactor replacement is not owner work |
| 74 | No 230 V at compressor — compressor voltage is not sensed when the compressor should be starting | A contactor stuck open, or a wiring error between the contactor and the compressor | Technician work: contactor and compressor-circuit diagnosis with the cabinet open |
| 81 | Low-stage thermal lockout — a thermal cutout has occurred in three consecutive low/high-stage cycles; low stage is locked out for 4 hours or until 24 V power is recycled | Whatever caused the repeated code 71 cutouts: a compressor repeatedly tripping its internal overload, or start components held in circuit | Do not just recycle power and re-lock it: the repeated overload trips need a technician's compressor diagnosis before the stage is run again |
| 82 | High-stage thermal lockout — a thermal cutout has occurred in three consecutive cycles; high stage is locked out for 4 hours or until 24 V power is recycled | The code 72 family, repeated: internal overload trips or start-component faults on high stage | Technician compressor diagnosis, as for code 81 |
| 83 | Low pressure lockout — a low pressure switch trip has occurred during three consecutive cycles; unit operation is locked out for 4 hours or until 24 V power is recycled | The code 32 family repeated three cycles running: low charge, a restriction, or indoor airflow too low (in heating, a heavily frosted outdoor coil) | Fix the 32 cause first (filter/airflow, then certified charge and leak work). Recycling power without the repair just re-enters the lockout count |
| 84 | High pressure lockout — a high pressure switch trip has occurred during three consecutive cycles; unit operation is locked out for 4 hours or until 24 V power is recycled | The code 31 family repeated three cycles running: dirty/restricted outdoor coil, outdoor fan not moving air, or overcharge | Clean the outdoor coil and confirm fan operation, then a technician verifies charge and the switch. Do not defeat the lockout — it is the compressor's last protection |
How to reset / clear error codes on Carrier heat pumps
Carrier's pressure codes (31/32) are self-managing before they are lockouts: the control de-energises the compressor contactor, runs the outdoor fan for up to 15 minutes, and retries the compressor every 15 minutes while demand exists; cycling the low voltage defeats the 15-minute delay. Three consecutive cycles with the same trip convert it to a lockout (81/82 thermal, 83 low pressure, 84 high pressure) that holds for 4 hours or until 24 V power is recycled. Sensor, brownout and voltage codes clear themselves when the measured condition returns to range. A power reset clears a lockout's timer, never its cause — a lockout that is only reset re-enters the same three-cycle count.
How to diagnose any Carrier 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.
Defrost on an Infinity heat pump is a sensor decision — and sensor failure quietly changes the rules
An Infinity heat pump does not defrost on a kitchen timer. The outdoor control watches two thermistors — the outdoor air temperature sensor (OAT) and the outdoor coil temperature sensor (OCT) — and defrost decisions are made from what those two readings say about frost accumulating on the coil. That is why the sensor codes 53 (OAT fault), 55 (OCT fault) and 56 (thermistors out of range, meaning the two readings no longer agree with each other) matter far beyond their modest descriptions: they are defrost codes in disguise. Carrier's installation instructions spell out the fallback behaviour for each failure, and it is worth knowing before a technician visit. If the OAT sensor fails, low-ambient cooling is not allowed, the one-minute outdoor fan off-delay does not occur, and defrost is initiated based on coil temperature and time. If the OCT sensor fails, low-ambient cooling is again blocked, and defrost changes character completely: it occurs at each time interval during heating operation but terminates after five minutes, whether or not the coil is actually clear. A thermistor-out-of-range error (56) produces the same timed defrost with the same five-minute termination. In other words, a Carrier with a dead coil sensor still defrosts — but blindly, on schedule, and briefly. The homeowner symptoms of that state are distinctive: defrosts that arrive like clockwork regardless of frost, steam puffs on mild days when no frost exists, and on genuinely icy days a coil that never quite clears because five minutes was not enough. None of those raise a 'defrost fault' code, because the Infinity table has none — the only fingerprints are 53, 55 or 56, plus the failed-thermistor default behaviour the manual documents. Two practical consequences follow. First, a sensor code on a heat pump is never a 'clear it and see' code in winter: the unit keeps heating, but its frost management has silently degraded, and the pressure codes (31/32) that heavy icing eventually produces will point at the refrigerant circuit when the real failure is a thermistor. Second, sensor placement is part of the specification: the manual requires the OAT thermistor locked in place with its spherical nib facing the front of the control box, and the OCT thermistor secured tight on the coil stub tube — a sensor hanging loose reads the wrong air, and code 56 is the control noticing that the two sensors' story no longer makes sense.
Carrier's lockout ladder: the 15-minute retry, the three-cycle rule, and what auxiliary heat is doing meanwhile
Carrier's outdoor control is patient before it is stubborn, and understanding its patience explains most 'it stopped, then it ran again, then it stopped for good' stories. A single high- or low-pressure trip (31 or 32) does not lock anything out: the control de-energises the compressor contactor, keeps the outdoor fan energised for up to 15 minutes, and — if demand still exists — restarts the compressor after that delay, repeating the attempt every 15 minutes while the fault persists. Cycling the low voltage defeats the delay, which is why a breaker flip seems to 'fix' the unit instantly: it has only skipped the queue. The escalation is by cycles, not by time. If the same trip occurs in three consecutive cycles, the control converts it to a lockout — 83 for low pressure, 84 for high pressure, 81/82 for the low/high-stage thermal cutouts (compressor operation detected, then disappearing, usually an internal overload trip) — and the affected operation is locked out for four hours or until 24 V power is recycled. During all of this the indoor side carries on: the thermostat, seeing an unsatisfied demand the outdoor unit cannot answer, stages auxiliary heat, which is why a house can stay warm for days on a locked-out heat pump while the electricity bill quietly doubles. That is the seasonal signature to watch for. A pressure lockout that begins with the first hard frost often traces to heating-mode causes Carrier lists for code 29/31 events — in heating, a dirty indoor coil or filter, or an indoor blower not running, raises head pressure just as surely as a filthy outdoor coil does in summer — while a summer 84 is the outdoor coil and fan story. The brownout code (46) belongs in the same mental file: voltage below 187 V for four seconds stops the compressor and fan until voltage recovers above 190 V, so a unit that 'trips codes' only when the whole neighbourhood's air conditioners start on a hot afternoon may be reporting the supply, not itself. The diagnostic discipline Carrier's table rewards is counting: note whether you are seeing a trip (31/32, self-retrying) or a lockout (83/84, three cycles deep), because the lockout has already told you the fault is consistent, and consistent faults get fixed, not reset. Two more Carrier behaviours round out the winter picture. The control also watches the compressor contactor itself: code 73 means the compressor has voltage with no demand present — a welded contactor — while 74 is the reverse, demand present with no 230 V reaching the compressor, and 47 extends that to no 230 V at the unit at all; between them, the table separates a supply problem, a switching problem and a load problem that owners experience identically as 'the outdoor unit will not run'. And Carrier's packaged Infinity heat pump (50CR) documents the brownout idea on the low-voltage side instead — secondary voltage sagging under 15 VAC for four seconds, clearing above 17 VAC — a reminder that even within one brand, the same word 'brownout' is measured at different points on different products, and the model's own manual, not the family's reputation, is the authority.
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Facts on this page were checked against manufacturer documentation: mans.io · www.manualslib.com · www.manualslib.com