Equipment error code

Carrier code 33

Limit circuit fault — the control detected an open limit or rollout switch in the heating sequence, or the limit circuit failed its continuity check when the board expected it closed.

Equipment: furnace
Brand: Carrier
Source checked: 2026-03-01

What this code means

Limit circuit fault — the control detected an open limit or rollout switch in the heating sequence, or the limit circuit failed its continuity check when the board expected it closed.

Source: Carrier 59MN7 Service Instructions — Fault Code Definitions (099-35-04PY-0A) · checked 2026-03-01

Which models show it

Carrier/Bryant 59MN7, 59TN6, 59SC5A, 59SP5A, and Infinity 98 gas furnaces with the integrated control fault display; also shown on some 58MCA/58MVP series with LED or display codes mapped to fault 33.

What is actually happening

The furnace started a heat cycle but shut down because a safety switch in the limit circuit opened, or the control cannot verify that circuit is intact. Heat stops even though the thermostat still calls for heat.

Is it safe to keep running?

Do not keep calling for heat while code 33 is active. The limit opened for a reason — often restricted airflow or an overheating heat exchanger.

Shut it down when

Shut the furnace off at the wall switch if you smell hot metal or burning, see soot, hear repeated hard lockouts, or the carbon monoxide alarm sounds.

What a homeowner can safely check

Confirm the thermostat is in Heat mode and the filter is clean — replace if you cannot see light through it.
Verify all supply and return grilles are open and unblocked.
Check that the blower access door is fully seated; many units will not run with the door ajar.
On high-efficiency units, look for a blocked condensate drain or full trap — some Carrier controls map condensate faults into limit-related lockouts.
Reset power once after fixing airflow; if code 33 returns within one cycle, stop resetting.

When to stop

Code returns after one clean filter and grille check.
You smell gas, burning plastic, or exhaust in the living space.
The furnace short-cycles more than twice in an hour.
You are tempted to bypass a limit or rollout switch — that is never a DIY fix.

Likely causes ranked

1. Restricted airflow (dirty filter, closed registers, undersized return)
How a tech confirms: Measures temperature rise across the furnace against the nameplate range and static pressure across the filter and coil.
Typical repair: $150–$350 · Air Conditioner Repair cost guide
2. Open high-limit or rollout switch from repeated overheating
How a tech confirms: Ohms out limit and rollout switches in the limit string; inspect heat exchanger for cracks or hot spots with a camera.
Typical repair: $200–$550 · Furnace Replacement cost guide
3. Weak blower motor or failed blower capacitor
How a tech confirms: Verifies blower RPM and amp draw on heat; tests capacitor microfarads and motor windings.
Typical repair: $250–$750 · Furnace Replacement cost guide
4. Blocked flue or failed inducer preventing proper venting
How a tech confirms: Measures combustion air and flue draft; inspects inducer wheel and pressure switch tubing.
Typical repair: $300–$900 · Furnace Replacement cost guide

Is this the end of this unit?

If a cracked heat exchanger is confirmed, replacement is usually recommended — especially past 15–18 years or when the repair quote exceeds about one-third of a new furnace install. See the furnace replacement cost guide for full-system pricing.

How to clear the code

Turn off power at the furnace switch or breaker for 30 seconds, restore power, and ensure the thermostat is calling for heat below its setpoint. On Infinity controls, use the service menu only if you have the homeowner guide — otherwise power cycle once.

Why clearing is not fixing it: Code 33 clears when the limit circuit closes again, but the underlying overheating or airflow problem remains. Clearing without fixing airflow often leads to a harder lockout or a rollout trip.

Which trade to call

Call a licensed hvac contractor who works on furnace equipment. Gas, refrigerant, and combustion faults are not general handyman jobs.

More codes from this brand

Symptom guide: Furnace Not Heating

FAQ

What does Carrier furnace code 33 mean?

Code 33 is a limit circuit fault: the control detected an open limit or rollout switch, or failed to see a closed limit circuit during startup. It is a safety shutdown, not a thermostat glitch.

Can a dirty filter cause Carrier code 33?

Yes. Low airflow is the most common homeowner-fixable trigger. Replace the filter, open registers, and power cycle once. If the code returns on the next heat cycle, schedule HVAC service.

How much does it cost to fix Carrier code 33?

Simple airflow service calls run $150–$350. Limit or rollout switch replacement with inspection is often $200–$550. Heat exchanger failure pushes toward $3,500–$7,500 furnace replacement.

Is code 33 the same as a cracked heat exchanger?

Not always, but repeated code 33 trips after a clean filter warrant a heat exchanger inspection. A cracked exchanger can trip rollout switches and is a carbon monoxide risk.

Should I keep resetting code 33 to get heat?

One reset after fixing airflow is reasonable. Repeated resets without service can damage the heat exchanger and void warranties. Stop after two failed cycles.

Source

Source — Carrier 59MN7 Service Instructions — Fault Code Definitions (099-35-04PY-0A). Checked 2026-03-01.