Carrier Furnace 33 Error Code: Meaning, Causes & Fix

Limit circuit fault — a limit, flame rollout, draft safeguard or blocked-vent switch (as fitted) in the limit circuit is open right now, but the control hasn't escalated to lockout (13) yet.

33 is 13's earlier chapter, and Carrier's service instructions define it by the circuit, not just by overheating: a limit OR flame rollout switch is open. When it opens, Carrier's control runs the blower for 4 minutes or until the open switch remakes, whichever is longer — the blower is cooling the exchanger down. If the switch stays open longer than 3 minutes, the code changes to lockout 13; if it remakes sooner, 33 keeps flashing until the blower shuts off. The high-limit branch is the airflow story (the furnace made heat faster than the blower could carry it away). The flame rollout branch is different and more serious: a rollout switch opens when flame escapes the burner compartment, it requires manual reset, and Carrier lists inadequate combustion air supply, a restricted vent and vent sizing among its causes — a tripped rollout is never just a reset-and-run event. Carrier's own check list for 33 also includes a loose blower wheel, a defective blower motor or capacitor, a defective switch or its connections, excessive wind, and a dirty filter or restricted duct system. Code 33 is where Carrier's flash-code system shows its real design: one code guards one circuit, and the circuit contains more than one kind of danger. The high limit watches temperature — the slow, airflow-driven danger of a furnace cooking its heat exchanger. The rollout switch watches flame position — the fast, combustion-driven danger of fire where fire must never be. Both open the same circuit and flash the same 33 because the control's first duty is identical for both: burners off, blower on, heat carried away. The homeowner's work is telling the two apart, and Carrier hands you the distinguishing tool for free: time and temperature. The limit switch remakes as things cool; the rollout switch waits for a human hand and a found cause. A 33 read with that distinction in mind is a diagnosis half-finished before a single panel is opened; a 33 treated as 'the overheating code' risks resetting the one switch in the furnace that exists to be investigated, not reset.

What causes the 33 code on a Carrier furnace?

Restricted airflow

Filter, closed registers, matted blower wheel or indoor coil — the overwhelming majority of high-limit openings.

Blower not keeping up

Failing motor/capacitor, a loose blower wheel, or wrong speed setting — all on Carrier's own 33 check list.

Flame rollout switch open

Flame escaping the burner area or inadequate combustion air — a combustion/venting fault, not an airflow nuisance; the switch requires manual reset and the cause must be found by a technician.

Restricted vent or vent sizing

Carrier lists a restricted vent, improper vent sizing and excessive wind for 33 — the vent path is part of this code's story, not only the pressure-switch codes'.

Limit switch drifting

Only after airflow is proven with a temperature-rise measurement.

Flame rollout: the branch of 33 that is not about airflow at all

Carrier's service instructions name the limit circuit's other resident alongside the high limit: the flame rollout switch. It sits where it can sense flame escaping the burner compartment — flame that should be drawn into the heat exchanger rolling out toward the controls instead — and when it opens, the same code 33 flashes for a completely different disease: blocked or restricted venting, inadequate combustion air supply, burner or heat-exchanger trouble. Carrier's instructions add the fact owners most need: the flame rollout switch requires manual reset. It does not cool down and remake like a high limit. A 33 that survives hours of cooling with the blower long stopped is telling you a manual-reset switch in the circuit is open — and rollout is a fire-and-carbon-monoxide conversation that belongs to a licensed technician, full stop.

A restricted vent or the wrong vent, strangling the furnace from the exhaust side

Homeowner folklore files vent problems under the pressure-switch codes, but Carrier's own 33 checklist includes restricted vent and proper vent sizing — and the mechanism is straightforward: a furnace that cannot exhaust freely runs its burner compartment and heat exchanger hotter, combustion air arrives inadequately, and either the high limit opens on temperature or the rollout switch opens on flame behaviour. An older furnace that begins tripping 33 after vent work, after a roof or siding change near the termination, or with a new higher-output unit on an old vent, is reciting this cause; it is installation-and-vent territory, and it is technician work with the vent inspected end to end.

The blower side failing quietly: loose wheel, tired motor, dead capacitor

Carrier lists loose blower wheel, defective blower motor and defective capacitor for 33, and together they describe airflow loss that a filter change cannot touch: the motor runs, the wheel slips on its shaft or its capacitor fades, and the furnace receives a fraction of its design airflow while sounding almost normal through the ducts. The presenting pattern is a furnace that heats in shorter and shorter bursts as the call goes on — each burst ending in 33 — because marginal airflow manages a cool furnace and fails a heat-soaked one. This is why a 33 that worsens through a single evening's heating call, on a clean filter with open registers, points at the blower assembly rather than at the ductwork.

Excessive wind and the marginal installation

Carrier also names excessive wind for code 33, which surprises owners until the mechanism is clear: strong wind at the vent termination disturbs draft and flame stability, and a marginal vent or combustion-air arrangement that behaves in calm weather can push flame toward the rollout switch or heat toward the limit on the windiest nights. A 33 whose appearances correlate with storms and calm-day normality is not an intermittent electrical ghost; it is a vent-and-wind diagnosis, and the fix lives at the termination and vent sizing, which is professional territory.

Checks to run before you replace anything

1. Replace the filter; open all registers/returns.
2. Watch whether the burners relight after the blower cools the furnace (normal high-limit 33 behaviour: blower runs about 4 minutes) — and count how often it cycles this way. A 33 that arrives with a burning smell, scorch marks or visible flame disturbance is the rollout branch: power off and call a technician, do not keep watching it cycle.
3. Power-off inspection of the blower wheel for matting and for looseness on its shaft (Carrier lists a loose blower wheel for this code).
4. Read the flash pattern properly before anything else: on Carrier's two-digit system the code flashes as 3 short flashes, a pause, then 3 long flashes for 33 — the pattern behind the common search 'Carrier furnace error codes 3 short 3 long'. Count a full cycle twice or video it, because a miscounted pattern (31, 13, 34) sends the whole diagnosis to the wrong system.
5. Use Carrier's timing as a free test: when the limit circuit opens, the control runs the blower for 4 minutes or until the open switch remakes, whichever is longer. A switch remaking within about 3 minutes lets 33 clear as the blower shuts off; an opening that lasts longer than 3 minutes converts the code to lockout 13. Watching which of those your furnace does — and how many times in one heating call — separates an occasional high-limit event from a circuit that is effectively staying open.
6. Apply the cooling test to sort limit from rollout without opening anything: let the furnace sit powered but idle for a good while after a 33. A high-limit 33 is gone once everything is cold — the switch remade as it cooled. A 33 that is still present on a stone-cold furnace has a switch in the circuit that does not self-reset: the flame rollout switch (manual reset, per Carrier) or a failed switch or connection. Report that distinction to the technician verbatim; it changes what they bring and what they check first.
7. Do the homeowner-legitimate airflow pass completely, not partially: replace (not inspect) the filter — and check its rating, because an overly restrictive high-MERV filter fitted recently is a self-inflicted 33 — open every supply register and every return, confirm nothing blocks the return grille, and note whether 33 still trips on a long call. If it does, airflow at the furnace itself (blower wheel, motor, capacitor, indoor coil) is the remaining airflow territory, and it is a technician's measurement job from there.

How to fix error code 33 carrier furnace?

1. Restore airflow (filter, registers, wheel/coil cleaning).
2. Blower motor/capacitor service if airflow is weak with a clean path.
3. Limit switch replacement only on a proven temperature-rise measurement.
4. A tripped flame rollout switch is reset manually only after a technician has found why flame left the burner compartment (vent, combustion air, burner or heat-exchanger cause) — resetting a rollout switch without that diagnosis is defeating a fire guard.
5. Restore house-side airflow and then demand a full-length proof: a furnace that survives its longest, coldest-evening heating call without a 33 has been fixed; one that merely survives ten minutes has been postponed. Marginal restrictions pass short tests, which is why 33 repairs are verified by runtime, not by a quick relight.
6. Treat any rollout involvement as a hard stop for DIY: if the cold furnace still shows 33, if there is any burning smell, scorching or soot near the burners, or if a technician confirms the rollout switch tripped, the furnace stays off until the rollout cause — venting, combustion air, burners, heat exchanger — is found and corrected by a licensed HVAC technician, and only then is the manual-reset switch reset. Resetting a rollout switch to 'see if it happens again' is experimenting with flame outside its compartment.
7. Let temperature rise arbitrate the switch question: a technician measures supply-minus-return temperature rise across the furnace and compares it with the range on the rating plate. Rise above the plate range with airflow restored means the furnace is genuinely overheating on a deeper restriction or a blower problem; rise inside the range with 33 still tripping is the rare case where the limit switch itself has drifted and earns replacement. Either measurement ends the argument that filters and opinions cannot.

The part that usually fails: None usually — airflow components (filter/wheel/coil)

Temperature rise vs the rating plate is the arbiter, as with 13.

Carrier furnace error code 33 reset

It depends which switch opened. A high-limit opening clears itself: the switch remakes as the furnace cools (blower running about 4 minutes), and 33 stops once the blower shuts off. A flame rollout switch does NOT re-close on its own — Carrier's instructions state it requires manual reset, and that reset belongs after the rollout cause is diagnosed. Power-cycling the furnace hurries neither and fixes nothing.

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.

Why does the 33 code keep coming back?

33s that grow more frequent = a restriction still growing (coil matting is the slow classic) or a blower declining. A returning 33 should be read as a trend line, not a series of accidents. Trips that cluster on the coldest nights are margin trips — airflow or venting that copes until the furnace runs continuously — and they worsen as the underlying restriction grows, which it always does in one direction: filters load, coils mat, blower capacitors fade. Trips that migrate from long calls to short ones over a season are the same story further along. And any 33 that returns after a rollout reset, or arrives with odour, noise or visible flame disturbance, has left the trend-line category entirely: that furnace stays off until a licensed technician has inspected the burner compartment, vent and heat exchanger, because repeated flame rollout is how heat exchangers announce the failures that produce carbon monoxide.

How to stop 33 coming back

• Restricted airflow: Filter, closed registers, matted blower wheel or indoor coil — the overwhelming majority of high-limit openings.
• Blower not keeping up: Failing motor/capacitor, a loose blower wheel, or wrong speed setting — all on Carrier's own 33 check list.
• Flame rollout switch open: Flame escaping the burner area or inadequate combustion air — a combustion/venting fault, not an airflow nuisance; the switch requires manual reset and the cause must be found by a technician.

Retrieving stored fault 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.

Other Carrier furnace error codes

See the full list on the Carrier Furnace error codes hub.

Codes 33 gets confused with

13 — Limit circuit lockout — the control locked the furnace out because a limit or flame rollout switch in the limit circuit stayed open longer than 3 minutes, or limit trips came in succession (10 successive trips during high-heat; the variable-speed generation also locks out after 3 successive trips during low-heat). The control auto-resets after 3 hours. 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.

43 — Low-heat pressure switch open while the high-heat pressure switch is closed — the stage switches disagree. That is the mainstream Carrier chart's wording (58CVA service label: 'LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED'). On the older 58MVP variable-speed generation the same code is titled PRESSURE SWITCH CALIBRATION FAULT — see the honesty note in the what and FAQ: both titles are Carrier's own, on different generations. A two-stage Carrier furnace proves its vent with two pressure switches set at different drafts: the low-heat switch makes at the gentle draft of low fire, and the high-heat switch needs the stronger draft of high fire before it will make.

21 — Gas heating lockout — Carrier's service label words it: 'STATUS CODE 21 — GAS HEATING LOCKOUT. Control will NOT auto reset. Check for: mis-wired gas valve; defective control (valve relay).' The control has shut down gas heating and, uniquely among Carrier's common lockouts, it will not clear itself with time. Code 21 is the Carrier control declaring that it cannot trust its own gas valve output.

31 — Pressure switch did not close (or opened) during operation — the inducer couldn't prove proper venting draft. Before Carrier furnaces light, the inducer fan runs and the pressure switch must close to prove the vent is drafting.

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