What causes Carrier Gas Furnace 33?

Dirty air filter or restricted return-air grilles and ductwork, the first item on Carrier's check list. Too many supply registers closed, undersized ducts, or high external static pressure. Blower problems: a loose blower wheel on the motor shaft, a failing blower motor, or a bad capacitor. Furnace overfired: gas input or inlet pressure set too high, wrong orifices, or a stuck high-heat solenoid in the gas valve. Blower speed too low for the temperature rise listed on the rating plate. Bypass humidifier or zoning bypass damper that is oversized, or modulating zone dampers starving the furnace of airflow. Flame rollout switch tripped by inadequate combustion air, misaligned burners, a blocked heat exchanger, or a heat exchanger leak. A weak limit switch that opens below its rated temperature, a missing or misaligned limit shield or baffle, or an open red wire or bad terminal in the limit circuit.

How do I fix Carrier Gas Furnace 33?

Confirm the code: look through the blower door sight glass and count 3 short flashes then 3 long flashes. Write it down before cutting power, since a power cycle clears the code history. Replace or clean the air filter. Carrier's checkout says to clean or replace the filter and recycle the furnace after the limit resets. Open every supply register and make sure return grilles aren't blocked by furniture, rugs, or stored boxes. Turn power off at the furnace switch, remove the blower door, and check that the blower wheel is tight on the motor shaft and that the wheel isn't caked with dust. If you have a bypass humidifier, close its damper for a test run (it is an extra path the air bypasses through). Restore power and let the furnace run a full heating cycle with the doors on. If code 33 doesn't come back after a filter change, the filter was likely the cause. If the furnace still trips, or the flame rollout switch has tripped (the small switch near the burners with a reset button), don't just keep resetting it. A rollout trip means flames escaped the burner box, so call a pro.

When should I call a pro for Carrier Gas Furnace 33?

Code 33 keeps returning with a clean filter and open registers. The flame rollout switch has tripped. Carrier ties repeat rollout trips to inadequate combustion air, burner misalignment, a blocked heat exchanger, or a heat exchanger leak, which can mean carbon monoxide risk. Gas input needs clocking or adjusting, or the gas valve's high-heat solenoid may be stuck. The temperature rise needs measuring against the rating plate and blower airflow settings need changing. The limit switch needs testing with a temperature probe (Carrier replaces it if it opens 10°F or more below its setpoint), or the limit wiring and harness need repair. You smell gas: leave the house and call the gas utility. Never jumper or bypass the limit or rollout switch.

Code: 33🔥 Carrier Gas Furnace

Carrier Furnace Code 33: Limit Circuit Fault, Limit or Flame Rollout Switch Open (Meaning, Causes, Fixes)

Q: What does Carrier Gas Furnace 33 mean?

Status code 33 on a Carrier furnace (3 short flashes, then 3 long flashes) means the high-temperature limit switch or the flame rollout switch has opened. Most of the time the furnace is overheating from poor airflow, and a clogged filter is the first thing to check. If the switch stays open longer than 3 minutes, the code becomes lockout 13. Carrier's troubleshooting guides define status code 33 as a limit circuit fault: the limit switch (LS) or flame rollout switch (FRS) is open. Carrier status codes are two-digit numbers where the first digit is the number of short flashes and the second the number of long flashes, so code 33 is 3 short flashes followed by 3 long flashes. On two-stage variable-speed models it can also mean the furnace has dropped into high-heat-only mode after two low-heat limit trips in a row. When the circuit opens, the blower runs for at least 4 minutes (or until the switch closes again) to cool the heat exchanger. If the switch stays open more than 3 minutes, or trips 10 times in a row in high heat, the control shows lockout code 13 and auto-resets after 3 hours. The limit switch resets itself once it cools; the flame rollout switch is manual-reset and stays open until someone presses its button or replaces it.

Reviewed 2026-10-22 · Based on 3 manufacturer & service sources

What the code means

Carrier's troubleshooting guides define status code 33 as a limit circuit fault: the limit switch (LS) or flame rollout switch (FRS) is open. Carrier status codes are two-digit numbers where the first digit is the number of short flashes and the second the number of long flashes, so code 33 is 3 short flashes followed by 3 long flashes. On two-stage variable-speed models it can also mean the furnace has dropped into high-heat-only mode after two low-heat limit trips in a row. When the circuit opens, the blower runs for at least 4 minutes (or until the switch closes again) to cool the heat exchanger. If the switch stays open more than 3 minutes, or trips 10 times in a row in high heat, the control shows lockout code 13 and auto-resets after 3 hours. The limit switch resets itself once it cools; the flame rollout switch is manual-reset and stays open until someone presses its button or replaces it.

Likely causes

Dirty air filter or restricted return-air grilles and ductwork, the first item on Carrier's check list
Too many supply registers closed, undersized ducts, or high external static pressure
Blower problems: a loose blower wheel on the motor shaft, a failing blower motor, or a bad capacitor
Furnace overfired: gas input or inlet pressure set too high, wrong orifices, or a stuck high-heat solenoid in the gas valve
Blower speed too low for the temperature rise listed on the rating plate
Bypass humidifier or zoning bypass damper that is oversized, or modulating zone dampers starving the furnace of airflow
Flame rollout switch tripped by inadequate combustion air, misaligned burners, a blocked heat exchanger, or a heat exchanger leak
A weak limit switch that opens below its rated temperature, a missing or misaligned limit shield or baffle, or an open red wire or bad terminal in the limit circuit

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.

Confirm the code: look through the blower door sight glass and count 3 short flashes then 3 long flashes. Write it down before cutting power, since a power cycle clears the code history.
Replace or clean the air filter. Carrier's checkout says to clean or replace the filter and recycle the furnace after the limit resets.
Open every supply register and make sure return grilles aren't blocked by furniture, rugs, or stored boxes.
Turn power off at the furnace switch, remove the blower door, and check that the blower wheel is tight on the motor shaft and that the wheel isn't caked with dust.
If you have a bypass humidifier, close its damper for a test run (it is an extra path the air bypasses through).
Restore power and let the furnace run a full heating cycle with the doors on. If code 33 doesn't come back after a filter change, the filter was likely the cause.
If the furnace still trips, or the flame rollout switch has tripped (the small switch near the burners with a reset button), don't just keep resetting it. A rollout trip means flames escaped the burner box, so call a pro.

🛠 When to call a pro

Code 33 keeps returning with a clean filter and open registers.
The flame rollout switch has tripped. Carrier ties repeat rollout trips to inadequate combustion air, burner misalignment, a blocked heat exchanger, or a heat exchanger leak, which can mean carbon monoxide risk.
Gas input needs clocking or adjusting, or the gas valve's high-heat solenoid may be stuck.
The temperature rise needs measuring against the rating plate and blower airflow settings need changing.
The limit switch needs testing with a temperature probe (Carrier replaces it if it opens 10°F or more below its setpoint), or the limit wiring and harness need repair.
You smell gas: leave the house and call the gas utility. Never jumper or bypass the limit or rollout switch.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Furnace air filter in your exact size

The cheapest and most common fix. Check the size printed on the filter frame.

[affiliate link slot: AFFILIATE_SLOT_FILTER]

⚙

Digital multimeter

For continuity checks on the limit and rollout switches with power off.

[affiliate link slot: AFFILIATE_SLOT_MULTIMETER]

⚙

Limit switch (exact OEM part number for your model)

Only after airflow is verified. A switch that opens below its setpoint must be replaced with the same rated part, never a higher-temperature one.

[affiliate link slot: AFFILIATE_SLOT_LIMIT_SWITCH]

⚙

Flame rollout switch (exact OEM part number for your model)

Replace only if it won't reset, and only after a pro finds why it tripped.

[affiliate link slot: AFFILIATE_SLOT_ROLLOUT_SWITCH]

Sources

Last reviewed 2026-10-22. Always confirm against the manual for your exact model number.