Carrier Furnace 13 Error Code: Meaning, Causes & Fix

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. 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.

What causes the 13 code on a Carrier furnace?

Clogged air filter

The #1 cause of limit trips everywhere. A filter that looks 'not too bad' can be the whole story.

Blocked/closed registers or returns

Furniture over returns, most vents closed 'to force air elsewhere' — static pressure soars, airflow dies.

Blower problems

A failing blower motor, a slipping/dirty blower wheel, or a wrong blower speed tap — airflow below design.

Oversized furnace / undersized ducts

The installation mismatch that trips limits on the coldest days when runtimes are longest.

Failed limit switch (rare, and last)

Switches do drift, but condemning the switch before airflow is proven is the classic misdiagnosis.

Overfiring: too much gas input for the air available

Carrier's limit-code check lists include improper minimum- or maximum-heat gas input adjustment and, on variable-speed models, a stuck high-heat solenoid in the gas valve. A furnace burning more gas than its rating — after a gas valve change or a manifold pressure never set with a manometer — makes heat faster than even a healthy blower can shed it, and the limit trips on schedule: clean filter, open registers, still overheating. Setting gas input is licensed-technician work.

Excessive return-air restriction — the house starving the furnace

Carrier's code 13 flow ends at a check most owners never think of: excessive return-air restriction. The manual directs the technician to check all dampers, filters and return-air grilles for blockage and to add more return-air openings if necessary, evaluating external static pressure. A finished basement that sealed off a return, or one undersized return grille serving a larger furnace, starves the blower regardless of filter. Code 13 is often written by the return side.

The rollout branch reaching lockout directly

13 is not only the repeated-overheating lockout: Carrier's trigger includes a limit OR flame rollout switch open longer than 3 minutes, so one rollout event that stays open reaches 13 with no overheating history (on the 58CVA generation the circuit can also include a draft safeguard or blocked vent switch, likewise manual-reset). When Carrier's flow tests a rollout switch that re-trips after reset, it names inadequate combustion-air supply: reversed vent and combustion-air pipes, poor burner or orifice alignment, a blocked heat exchanger, or a leak between the heat exchangers. All licensed-technician diagnoses — and a 13 with scorch marks belongs to this branch until proven otherwise.

A limit switch or shield that is wrong, misaligned or drifting

Carrier's checks ask whether the furnace has the proper limit switch and limit shield fitted, and whether switch, shield and heat exchangers are properly aligned — a replacement switch with the wrong temperature rating, or a shield refitted crooked, changes the temperature the switch actually senses. This is the one branch where the switch itself is guilty, proven Carrier's way: continuity checks and observed heating cycles, not a switch swapped on suspicion.

Checks to run before you replace anything

1. 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.
2. Replace the filter (don't just inspect it — replace it) and open every register and return in the house.
3. Check the blower wheel through the blower compartment (power OFF at the switch and breaker): a wheel matted with dust moves a fraction of its rated air; the wheel and the evaporator coil above the furnace (if AC is fitted) are the hidden restrictions.
4. Note the pattern: trips on the coldest nights = capacity/airflow margin; trips at any time = worsening restriction or blower decline.
5. Learn the lockout arithmetic so you can describe your furnace accurately: on Carrier's current charts, 13 follows an opening longer than 3 minutes, or 10 successive limit trips during high-heat; the variable-speed label adds 3 successive trips during low-heat. 'It tripped about ten times on high fire, then locked out' is a diagnosis-grade observation.
6. Check the return side as deliberately as the supply side: every return grille uncovered, zone dampers open, nothing sealing a return path. Carrier's own final check for 13 is return-air restriction — the branch a filter change cannot fix.
7. Use the cold-furnace test to separate the branches: hours after the lockout, a high-limit 13 has remade its switch as everything cooled, while a 13 held by an open flame rollout switch is still open — Carrier states the rollout switch requires manual reset. Report a lockout that never clears even cold exactly that way.
8. Note what changed before the first lockout: a gas valve or limit switch replaced, a basement finished, a higher-MERV filter fitted, a larger furnace on old ducts. Code 13 is a margin fault; the change log usually names it.

How to fix error code 13 on carrier furnace?

1. Restore airflow: new filter, open registers, clean the blower wheel (and have the indoor coil cleaned if it's matted — HVAC technician work).
2. Correct blower speed/settings per the installation manual if a recent motor change mis-set them.
3. Replace the limit switch only after airflow is proven adequate (a technician verifies temperature rise across the heat exchanger against the rating plate — the definitive test).
4. Fix airflow at the system level, not just the filter: if return restriction is the finding, the repair is ductwork — opening or adding return capacity, correcting dampers — verified by an external static-pressure reading, the evaluation Carrier's flow specifies. A furnace that needs more return air locks out through every filter you buy.
5. Correct overfiring only with instruments: a technician sets gas input by manifold pressure and frees or replaces a stuck high-heat solenoid. Never judge gas input by flame appearance.
6. Prove the repair the way Carrier's flow proves it: repeated full heating cycles observed after the fix (Carrier's rollout test uses two 15-minute cycles) with temperature rise inside the rating-plate range. One short cycle that 'seems fine' is not proof for a lockout that took successive trips to declare itself.

The part that usually fails: Usually NO part — airflow is the fault (filter, blower wheel, coil)

The limit switch is the messenger and the furnace's fire guard. A technician measures temperature rise (supply minus return air temperature) and compares it with the range printed on the rating plate — that number, not opinion, decides whether the switch or the airflow is guilty.

How to reset / clear the 13 code

Code 13 lockout auto-resets after 3 hours (Carrier's stated timing — the 1-hour figure sometimes quoted belongs to code 45). Power can be cycled at the furnace switch once the cause is fixed, but resetting without fixing airflow just re-trips and re-locks. One exception: if the open switch is the flame rollout switch, it does not re-close on its own — Carrier requires it to be manually reset, and only after a technician has found why flame rolled out.

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 13 code keep 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.

How to stop 13 coming back

• Clogged air filter: The #1 cause of limit trips everywhere. A filter that looks 'not too bad' can be the whole story.
• Blocked/closed registers or returns: Furniture over returns, most vents closed 'to force air elsewhere' — static pressure soars, airflow dies.
• Blower problems: A failing blower motor, a slipping/dirty blower wheel, or a wrong blower speed tap — airflow below design.

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 13 gets confused with

33 — 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.

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.

Frequently asked questions

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