Carrier Furnace Error Codes List: Every Code Explained

Every Carrier furnace 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 furnaces

Carrier boards include a component test mode (a test pin/shorting jumper on the board, per the installation manual) that runs the inducer, blower and ignition sequence on demand, and the board keeps the last faults in memory readable as LED codes. The full fault chart is printed on the blower-door label of every unit — photograph it before relying on any online chart, because code assignments differ slightly between board generations.

The complete Carrier furnace error code list

How to reset / clear error codes on Carrier furnaces

Carrier fault codes are stored by the control board and shown as two-digit codes via the status LED (short flash = first digit, long flash = second digit, with a pause between). Most codes clear automatically once the fault condition is gone and a new call for heat succeeds; a hard lockout (such as code 13 or 14) needs the fault fixed, then power cycled at the furnace switch or breaker for a few seconds, and in some lockouts the board waits up to an hour or requires several successful cycles before normal operation resumes. Clearing a lockout without fixing its cause just restarts the countdown to the next lockout.

How to diagnose any Carrier furnace code

Work the problem in this order — and stop at any point where gas, combustion or mains wiring is involved, because those belong to a licensed HVAC technician: (1) record the code exactly: count LED flashes slowly, twice, or photograph the alphanumeric display; (2) check the free things homeowners legitimately own: the thermostat is calling for heat, the furnace power switch and breaker are on, the filter is not clogged solid (a starved furnace overheats and locks out on limit codes), all vents and the return are open, and the condensate drain is not blocked on condensing models; (3) reset once — at the power switch, off for 30 seconds — then watch a full call for heat from the beginning: inducer, ignition, flame, blower, in that order; where the sequence stops is the fault family the code names; (4) a code that returns immediately is an active fault — do not keep resetting a lockout, especially any flame or limit code; (5) everything inside the burner compartment — flame sensor cleaning is the one grey area many manuals allow owners, but gas valves, burners, heat exchangers and control boards are professional work. If you smell gas at any point, leave and call the gas utility, not a helpline.

How Carrier's two-digit blink codes work: short flashes first, long flashes second

Carrier furnaces report faults as a two-digit status code flashed on the amber LED at the control board, read through the blower-door sight glass. The first digit is the number of short flashes; the second digit is the number of long flashes; then the pattern repeats. Code 31 is three short flashes and one long flash — and there is no '1 long, 3 short' code, so a pattern remembered in that order is a code read from the middle of its cycle. The board also remembers: stored codes can be recalled in the order they occurred (on the 59MN7, disconnect the R thermostat lead, reset power and set setup switch SW1-1 ON), they are erased automatically after 72 hours, and they can be cleared manually by jumpering R, W/W1 and Y/Y2 simultaneously until code 11 flashes. Code 11 itself means 'no previous code' — an empty history, not a fault. A steady-on LED means the control has 24V power, a dark LED means it does not, and rapid flashing warns of reversed line-voltage polarity. One generation differs: the variable-speed 58MVP family splits the digits across two LEDs — yellow flashes count the first digit, green flashes count the second — so on those boards you count colours, not short and long. Whatever the mechanism, video the LED before resetting anything: a lockout cleared at the switch takes its evidence with it. The same boards can also run a component test (setup switch SW1-6 on many models) that exercises the inducer, igniter and blower and ends by flashing its result — 11 for a pass, or the fault code it found.

One furnace family, several code generations — and Infinity's second language

Carrier has reused this two-digit chart across decades of furnaces, and the assignments are not identical on every generation — which is why the chart printed inside your own blower door is the final authority for your furnace. The mainstream chart used by the 58MCB, 59SC2 and 59MN7 families reads: 21 gas heating lockout, 22 abnormal flame-proving signal, 23 pressure switch did not open, 25 invalid model selection or setup error. The older variable-speed 58MVP service label assigns those same numbers differently — 21 invalid model selection, 22 setup error, 23 invalid blower airflow selection — and its code 12 is a blower calibration lockout, not the mainstream 'blower on after power up' status. The pressure-switch pair shifts too: on the mainstream chart 31 covers the pressure-switch family, while variable-speed models split it — 31 for the high-heat switch, 32 for the low-heat switch. Communicating Infinity systems add a second reporting path rather than a second code list: the furnace's own faults surface at the wall control as system malfunction events carrying the same numbers (13 limit circuit lockout, 14 ignition lockout, 21, 22, 23, 24, 33, 41 and 45 among them). But the wall control can also display three-digit system events that are not furnace board codes at all — they appear in no two-digit chart on this page, and only Carrier's documentation for your specific Infinity control defines them. Match the number to the system that printed it before you diagnose it — and when you call a technician, quote the code together with the full model number from the rating plate, because that pair, not the number alone, identifies the fault.

Blink codes, display codes and lockouts

Two generations of fault reporting live side by side on modern furnaces. Older and simpler boards report through a single LED: a repeating count of flashes, read slowly against the chart printed inside the blower door — that chart, for your exact model, outranks any list on the internet, including this one. Newer communicating furnaces show alphanumeric codes (the E-series in the table above) and store a fault history a technician can scroll. Both generations share the lockout idea: after a set number of failed attempts at the same step — ignition is the classic — the board stops trying for a period (often an hour, or until power is cycled) instead of cycling forever against a fault that could overheat or flood the burners with unburned gas. A furnace in lockout is not broken by the lockout itself; it is a furnace that has already failed the same step repeatedly and is waiting for someone to find out why.

Furnace codes are also seasonal in a way washer codes are not. The first cold snap produces a wave of ignition and flame codes from furnaces that sat idle all summer — oxidised flame sensors, spider-nested burners, pressure-switch hoses holding condensate from the cooling season. Deep-winter adds frozen condensate lines on high-efficiency models and intake/exhaust terminations blocked by snow or ice, which present as pressure-switch codes. A code that appears once, on the coldest night of the year, and never again has often met its cause already; a code that appears every cycle, in any weather, is a component asking to be tested.

The most searched Carrier furnace codes, in detail

12 — Blower on after power-up

Carrier code 12 confuses owners because it appears in the fault-code list, but it describes designed behaviour: if power returns while a heat call is active, the control runs the blower briefly before proceeding — clearing any residual heat and proving the blower before ignition. If the furnace then heats normally, 12 was information. It only becomes a topic if the furnace never progresses past it.

Most likely cause: A power interruption during a heat call — The designed trigger — outage, breaker work, or someone flipped the furnace switch mid-cycle.

First check: Give it its ~90 seconds: normal sequence = blower runs, stops, then a normal ignition sequence follows if the call persists.

If it keeps coming back: Frequent power interruptions (not the code) are worth an electrician's glance if breakers are involved.

13 — Limit circuit lockout

Carrier code 13 is a lockout, not a first offence, and Carrier defines its triggers exactly: a limit or flame rollout switch (on some models also a draft safeguard or blocked vent switch in the same circuit) open longer than 3 minutes, or successive limit trips — 10 during high-heat, and on the variable-speed generation 3 during low-heat. The control then shuts the burners down and auto-resets after 3 hours. Behind almost every 13 is code 33's story: the furnace made heat faster than it could shed it, because the limit opened when too little air moved over the heat exchanger — airflow is the entire story until proven otherwise — or because flame rolled out, which is a combustion fault, not an airflow nuisance. A furnace that heats faster than it can shed heat is protecting itself; believe it. Read 13 in two clauses: code 33's clause — a switch in the limit circuit opened — and the escalation: the opening lasted longer than 3 minutes, or the trips came in succession — 10 during high-heat on Carrier's current charts, the variable-speed generation adding 3 successive trips during low-heat. Together they mean the furnace has stopped believing the problem is momentary, so it locks out and waits 3 hours. That is why 13 arrives on the coldest night: only continuous running produces successive trips. A 13 in a warm spell suggests the 3-minute branch, or a restriction so advanced it trips even on short calls.

Most likely cause: Clogged air filter — The #1 cause of limit trips everywhere. A filter that looks 'not too bad' can be the whole story.

First check: SAFETY: this is a gas appliance with an overheating history. If you smell gas, see scorching, or the furnace is banging/booming on ignition — power off, gas off if you know how, call a technician now.

If it keeps coming back: 13 returning after filter and registers are sorted means the restriction is deeper: matted evaporator coil, collapsing duct liner, a blower losing speed, or a genuinely oversized furnace. Each return trip is the heat exchanger taking thermal stress — don't let it become a habit. When 13 returns after a genuine airflow repair, read the interval as a gauge. Lockouts weeks apart, only on the coldest nights, mean the margin is still thin — return capacity or blower performance is borderline and worsens as the filter loads. Lockouts arriving daily, then within single calls, mean the restriction is progressing (matting coil, fading blower capacitor, collapsing duct liner) or the furnace is overfired. A 13 that follows a rollout reset is a different category: Carrier treats a re-tripping rollout switch as inadequate combustion air — reversed vent pipes, burner misalignment, a blocked or leaking heat exchanger — carbon-monoxide territory. That furnace stays off, and the next visit is a combustion inspection.

14 — Ignition lockout

14 is the lockout that follows code 34-style ignition failures: the control ran its trial-for-ignition sequence (inducer, igniter, gas valve, flame proof) three times without establishing flame, and shut down for safety. The failure inside those trials is the real fault — dirty flame sensor (if it lights then drops), a failed hot-surface igniter (never lights), gas supply issues, or a weak ignition process. 14 tells you the trials happened; the observation of a trial tells you why.

Most likely cause: Dirty flame sensor — If the burners light then cut within seconds, the sensor's oxide coat is the classic — the commonest ignition fault in all furnaces.

First check: SAFETY: repeated ignition trials with raw gas smell = stop, ventilate, call the gas utility/pro. Do not keep cycling it.

If it keeps coming back: 14 returning after a sensor clean in a few weeks means the sensor is genuinely failing or combustion is dirtying it fast (burner/combustion check). Returning after an igniter change suggests gas pressure or valve issues.

21 — Gas heating lockout

Code 21 is the Carrier control declaring that it cannot trust its own gas valve output. Carrier's variable-speed troubleshooting guide states the two branches plainly: the main gas valve relay (MGVR) on the furnace control is stuck closed, or there is a miswire or short to the gas valve wiring — either way, the gas valve could be energised when it should not be, or the control can no longer prove that it could not be. That is why 21 behaves differently from Carrier's other lockouts: codes 13 and 14 auto-reset after 3 hours because their causes (an overheated limit circuit, a failed ignition) can plausibly clear with rest, but a welded relay or a shorted wire does not heal, so 21 waits for a human and a power interruption instead. Read 21 against its neighbour 22 to keep the family straight: 22 (abnormal flame-proving signal) means flame is actually being proved while the gas valve is de-energised — gas is physically passing a valve that should be shut, and the inducer runs until the fault clears. Code 21 is one step earlier in the chain and purely electrical: the command path itself — the relay that sends power to the valve, or the wires that carry it — is suspect, whether or not any gas has moved. In practice 21 most often appears in two settings: immediately after a control or gas valve replacement (a miswire announcing itself at the first power-up — Carrier's guide names the terminal identities to check: the BLUE wire goes to the gas valve M terminal, the GREEN/YELLOW wire to the C terminal), or unprovoked in mid-season, when a relay that has switched the valve for years welds its contacts. Either way the furnace gives no heat, and no amount of waiting changes that.

Most likely cause: Gas valve relay stuck closed on the control — Carrier's first branch, in its own words: the main gas valve relay (MGVR) on the variable-speed furnace control is stuck closed. A relay that has switched an inductive valve load for thousands of cycles can weld its contacts; the control detects that its valve output cannot be trusted to be off and locks gas heating out rather than risk an uncommanded valve.

First check: SAFETY FIRST: 21 concerns the gas valve circuit. If you smell gas at any point, stop, leave the furnace off, and call your gas utility or a licensed HVAC technician. Never jumper, bypass or test-energise a gas valve to 'see if it works' — that defeats the exact safeguard this code is reporting on.

If it keeps coming back: A 21 that returns after the wiring has been proven correct, terminal by terminal, is the relay branch declaring itself intermittently — contacts that weld under load and free themselves when cold are exactly how this failure announces before it becomes permanent. That pattern ends at control replacement. A 21 that returns only after further work in the furnace is the harness being disturbed again — look where hands have been before condemning a second control.

24 — Secondary voltage fuse open

Code 24 means the little automotive-style fuse protecting the furnace's 24V control circuit has opened. The furnace will be dead or nearly so. The fuse is a symptom: something in the low-voltage circuit shorted — most classically a thermostat wire chafed through where it passes the cabinet or a shorted thermostat/contactor wire (in cooling season, the AC contactor circuit shares this transformer). Replacing the fuse without finding the short just blows the next one. Code 24 is the control protecting itself from its own wiring. The 24-volt secondary circuit powers everything the board commands — thermostat, pressure switches, gas valve, humidifier, outdoor contactor — and the small board fuse is the designated weak point, sized so a short anywhere in that web opens the fuse instead of cooking the transformer or the control. The consequence owners find frustrating: the code tells you the circuit failed, never which leg failed. Everything useful about a 24 is therefore in its timing and its mode, which is why Carrier's flow for this code is an isolation ritual — disconnect everything, restore calls one by one, and let the returning code accuse the guilty leg. On this code, method beats parts-swapping completely.

Most likely cause: Shorted thermostat wiring — Wires rub through at the cabinet entry or get nicked by screws — seasonal classic after any thermostat change.

First check: Power OFF. Pull the board fuse and check it (visibly blown or meter-open).

If it keeps coming back: Recurring fuse failures with no visible short suggest an intermittent (vibration/heat-dependent) short or a failing contactor coil — a technician's isolation test finds it in one visit. A 24 that returns after a 'fixed' short usually means the short was intermittent and the repair found its resting state, not its working state. Vibration shorts appear when the blower runs; heat shorts appear when a coil or contactor warms; outdoor-leg shorts appear after rain works into a damaged cable. Log each return with mode, minutes into the cycle and weather, and the pattern will name the leg more reliably than another box of fuses. The one return that changes the job entirely is a 24 with a fuse that tests good: Carrier's flow sends that straight to the control board, and no wiring hunt will cure it.

31 — Pressure switch did not close (or opened) during operation

Before Carrier furnaces light, the inducer fan runs and the pressure switch must close to prove the vent is drafting. Code 31 means that proof failed: the switch never closed (or dropped out) when it should. The chain is inducer → vent path → pressure switch tubing → the switch itself. A blocked vent (snow, nests, ice at the termination) is both common and the safety-relevant cause: the switch exists to stop the furnace firing into a blocked flue. Code 31 is the furnace refusing to fire on faith. The sequence it guards is absolute: inducer first, draft proven by the pressure switch, and only then ignition — so a 31 is never about flame, gas or heat; it is about air moving through a pipe. That is why its cause list reads like plumbing — vent sizing, sags, traps, drains, frost, wind — and why its most misread symptom is the switch chattering at start-up, which sounds electrical but is usually water moving in the vent. Read 31 as 'I cannot yet prove my exhaust goes somewhere safe': restore the air path, measure before replacing, and treat the switch as the witness, not the culprit, until the manometer says otherwise.

Most likely cause: Blocked or restricted vent/intake — Snow/ice at the PVC termination, nests, a sagging vent run holding condensate.

First check: SAFETY: never bypass a pressure switch or run the furnace with it jumpered — it proves your exhaust has somewhere to go.

If it keeps coming back: 31 returning in deep cold snaps points at termination icing or a condensate line that freezes; returning after rain points at vent water ingress/sag. The weather correlation is the diagnosis. A returning 31 keeps a calendar as much as a fault. Returns in deep cold point to termination frost or a freezing condensate run; returns after rain point to vent water ingress or a sag that fills only when driven rain gets in; returns that shorten their interval over a season point to a trap slowly clogging or an inducer slowly weakening. The return that should end DIY is a 31 with clear vents, a clear trap and sound tubing — from there the diagnosis is a manometer reading against the switch's printed rating, a technician's visit. What never belongs in the plan is defeating the switch: every return of 31 is the furnace declining to fire into an unproven vent, and that refusal is the safety system working.

Less common and model-specific codes

Search data shows owners also meet these codes on Carrier furnaces: 126, Furnace. They are not part of the core code list Carrier 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.

Deep-dive guides for the most searched codes

Related guides

Facts on this page were checked against manufacturer documentation: mans.io · mans.io · www.shareddocs.com · mans.io · www.manualshelf.com · manualsdump.com