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Carrier gas furnace code 33.1: Limit circuit switch open, blower running to cool the furnace

Gas furnace, Canada and United States

A limit or flame rollout switch is open. The blower runs for 4 minutes or until the switch closes again, whichever is longer. If the switch stays open longer than 3 minutes the code changes to the 13.1 lockout. On 80% furnaces the draft safeguard and blocked vent switch (where fitted) are in the same circuit.

This needs a licensed gas technician. There are a few safe checks you can do first, marked below. Anything inside the unit is a job for a professional.

This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.

What causes it

InteliSense display furnaces

  1. Not enough air moving through the furnace because of a dirty filter or a restricted duct system, including closed or blocked registers and grilles.

    Check and clean or replace the furnace filterYou can do this

    1. Turn off the electrical supply to the furnace.
    2. Open the filter cabinet door on the side or bottom of the furnace and slide the filter out, dirty side up so the dirt does not spill. If your filter is fitted somewhere else, ask your dealer how to reach it.
    3. If the filter is torn, replace it. A dirty disposable media filter is not cleaned; replace it with one of the same part number and size, with its airflow arrow pointing towards the blower.
    4. Wash a dirty washable filter in cold water, spraying it from the opposite side to the airflow. A mild liquid detergent is fine. Let it dry.
    5. Put the clean filter back, close the filter door and turn the power back on.
      Never run the furnace without a filter in place. Check the filter every four weeks and always use the size and type originally specified for your furnace.

    Check the supply registers and return-air grilles are openYou can do this

    1. Go round the house and check the supply registers, where warm air comes out, are open.
    2. Check the return-air grilles are open and not covered by furniture, rugs or stored items.
    3. Check the furnace filter as well. If the airflow is still poor, call your dealer for service.

    Book a licensed gas technician

  2. A flame rollout switch has tripped, which the troubleshooting guides link to an inadequate combustion air supply. It stays open until it is reset by hand.

    Reset the furnace's rollout switch onceYou can do this

    1. Write down the status light code before you turn anything off.
    2. Turn off the electrical supply to the furnace, then remove the main furnace door.
    3. Find the manual reset rollout switch on the burner box and press its button once to reset it.
    4. Refit the door and turn the power back on.
    5. If the switch trips again, shut the furnace down and call for service.
      Reset it once only. The switch trips when the furnace overheats around the burners, which points to a combustion air, gas pressure, burner or venting problem that a technician must find and correct. Never bypass or jumper the switch.

    Book a licensed gas technician

  3. A restricted or wrongly sized vent, or strong wind at the vent. On 80% furnaces this can open the draft safeguard or blocked vent switch.

    Check the vent and air intake pipes outside are clearYou can do this

    1. Find where the furnace vent pipe ends outside the house. A two-pipe (direct-vent) installation also has an air intake pipe ending outside.
      Check only what you can see from the ground. Do not climb onto the roof or use a ladder. If the vent ends on the roof or goes into a chimney, book a technician to check it.
    2. Check nothing is blocking or covering the ends of the pipes, in any season, and clear anything stored or growing around them.
    3. On a condensing furnace with a one-pipe installation, also check the combustion-air pipe next to the furnace is not blocked and nothing is crowding the air openings around the furnace.
    4. If a pipe is blocked inside, has holes or has come apart, do not run the furnace. Call your dealer.
      A blocked, holed or disconnected vent pipe can let toxic fumes into your home.

    Book a licensed gas technician

  4. A loose blower wheel, or a faulty blower motor or capacitor.

    Book a licensed gas technician

  5. A faulty limit switch or connection, or the gas input set wrongly.

    Book a licensed gas technician

InteliSense modulating furnaces

  1. Not enough air moving through the furnace because of a dirty filter or a restricted duct system, including closed or blocked registers and grilles.

    Shut off the gas to the furnace, then the powerYou can do this

    1. Set the thermostat to its lowest setting and its mode to OFF.
      If you smell gas, stop. Do not light any appliance, touch any electrical switch or use a phone in the building. Leave and call your gas supplier from a neighbor's phone.
    2. Close the manual gas valve in the gas pipe to the furnace.
    3. Only then turn off the electrical supply to the furnace. Carrier says to shut off the gas first whenever the gas does not shut off or the furnace overheats.
    4. Leave the furnace off with its doors on and call your dealer for service.

    Book a licensed gas technician

  2. A flame rollout switch has tripped, which the manual links to an inadequate combustion air supply. A technician must correct the underlying problem before resetting it.

    Same check as above: Shut off the gas to the furnace, then the power

    Book a licensed gas technician

  3. A restricted or wrongly sized vent, or strong wind at the vent.

    Same check as above: Shut off the gas to the furnace, then the power

    Book a licensed gas technician

  4. A loose blower wheel, or a faulty blower motor.

    Same check as above: Shut off the gas to the furnace, then the power

    Book a licensed gas technician

  5. A faulty limit switch or connection, or the gas input set wrongly.

    Same check as above: Shut off the gas to the furnace, then the power

    Book a licensed gas technician

How to read this code on your unit

The three-digit display sits on the furnace control behind the blower door. Through the sight glass in the door you see only the status light, which flashes the first two digits of the code. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 3 long flashes is a 33.x code. Write down what you see before you open the door or switch the furnace off. The full code with its third digit shows on the control display only with the door removed and the door switch closed, which is a job for a technician. A technician can also look back at the last 7 faults in the FLt menu. Stored codes are erased by themselves after 72 hours of powered operation after the last fault, and the display shows non when no fault is stored.

Booking a licensed gas technician

Book a licensed gas technicianlicensed gas technician

  1. Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
  2. Note the code, how often it appears, and the model and serial numbers from the rating label.
  3. Book a licensed gas technician and give them those details.

Models that show this code

148 models in 12 ranges
RangeModels
58SB0B/58SB1B58SB0B045M14-12, 58SB0B045M17-14, 58SB0B070M14-12, 58SB0B070M17-12, 58SB0B070M17-16, 58SB0B070M21-16, 58SB0B090M17-14, 58SB0B090M21-16, 58SB0B090M21-20, 58SB0B090M24-20, 58SB0B110M21-20, 58SB0B110M24-20, 58SB0B135M24-20, 58SB0B155M24-20, 58SB1B045M14-12, 58SB1B045M17-14, 58SB1B070M14-12, 58SB1B070M17-12, 58SB1B070M17-16, 58SB1B070M21-16, 58SB1B090M17-14, 58SB1B090M21-16, 58SB1B090M21-20, 58SB1B090M24-20, 58SB1B110M21-20, 58SB1B110M24-20, 58SB1B135M24-20, 58SB1B155M24-20
58SC0B/58SC1B58SC0B045M14-12, 58SC0B045M17-14, 58SC0B070M14-12, 58SC0B070M17-12, 58SC0B070M17-16, 58SC0B070M21-16, 58SC0B090M17-14, 58SC0B090M21-16, 58SC0B090M21-20, 58SC0B110M21-20, 58SC0B110M24-20, 58SC0B135M24-20, 58SC1B045M14-12, 58SC1B045M17-14, 58SC1B070M14-12, 58SC1B070M17-12, 58SC1B070M17-16, 58SC1B070M21-16, 58SC1B090M17-14, 58SC1B090M21-16, 58SC1B090M21-20, 58SC1B110M21-20, 58SC1B110M24-20, 58SC1B135M24-20
58SP0B/58SP1B58SP0B045V14-12, 58SP0B070V14-12, 58SP0B070V17-16, 58SP0B090V17-16, 58SP0B090V21-20, 58SP0B110V21-22, 58SP0B135V24-22, 58SP1B045V14-12, 58SP1B070V14-12, 58SP1B070V17-16, 58SP1B090V17-16, 58SP1B090V21-20, 58SP1B110V21-22, 58SP1B135V24-22
58TN0B/58TN1B58TN0B045C17-12, 58TN0B070C14-12, 58TN0B070C17-16, 58TN0B070C21-20, 58TN0B090C17-16, 58TN0B090C21-20, 58TN0B110C21-20, 58TN0B135C24-22, 58TN1B045C17-12, 58TN1B070C14-12, 58TN1B070C17-16, 58TN1B070C21-20, 58TN1B090C17-16, 58TN1B090C21-20, 58TN1B110C21-20, 58TN1B135C24-22
58TP0B/58TP1B58TP0B045V14-12, 58TP0B070V14-12, 58TP0B070V17-16, 58TP0B090V17-16, 58TP0B090V21-20, 58TP0B110V21-22, 58TP0B135V24-22, 58TP1B045V14-12, 58TP1B070V14-12, 58TP1B070V17-16, 58TP1B090V17-16, 58TP1B090V21-20, 58TP1B110V21-22, 58TP1B135V24-22
59SC2E59SC2E040M14-12, 59SC2E040M17-12, 59SC2E060M14-12, 59SC2E060M17-14, 59SC2E080M17-16, 59SC2E080M21-20, 59SC2E100M21-20, 59SC2E100M21-22, 59SC2E120M24-20
59SC6A59SC6A026M14-10, 59SC6A040M14-10, 59SC6A040M17-12, 59SC6A060M14-12, 59SC6A060M17-16, 59SC6A080M17-16, 59SC6A080M21-20, 59SC6A100M21-20, 59SC6A100M21-22, 59SC6A120M24-22, 59SC6A140M24-22
59TN6C59TN6C040C14-10, 59TN6C060C17-14, 59TN6C080C17-14, 59TN6C080C21-20, 59TN6C100C21-22, 59TN6C120C24-22
59TN7A59TN7A060C17-16, 59TN7A080C17-16, 59TN7A080C21-20, 59TN7A100C21-22, 59TN7A120C24-22
59TP6C59TP6C040V14-10, 59TP6C040V17-12, 59TP6C060V14-12, 59TP6C060V17-16, 59TP6C080V17-16, 59TP6C080V21-20, 59TP6C100V21-20, 59TP6C100V21-22, 59TP6C120V24-22
59TP7A59TP7A060V17-16, 59TP7A080V17-16, 59TP7A080V21-20, 59TP7A100V21-20, 59TP7A100V21-22, 59TP7A120V24-22
59MN7C59MN7C060C17-14, 59MN7C060C21-20, 59MN7C080C17-14, 59MN7C080C21-20, 59MN7C100C21-22, 59MN7C120C24-22

Where this comes from

Checked against the manufacturer's manual on 11 October 2026.

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Other Carrier gas furnace codes

Code Meaning What you can do
13 Limit circuit lockout (depends on model) Safe checks first, then a pro
26 Burner thermal switch open Call a pro
33 Limit circuit fault, a limit or rollout switch is open Safe checks first, then a pro
13.1 Limit circuit lockout, a limit or rollout switch stayed open Safe checks first, then a pro
26.1 Burner thermal switch open, permanent lockout Call a pro
26.2 Burner thermal switch open, permanent lockout Call a pro
31.7 High heat only, high switch failed to close in time Safe checks first, then a pro
32.1 Main pressure switch (LPS) open Safe checks first, then a pro
32.2 Low pressure switch open Safe checks first, then a pro
33.2 High heat only for the current cycle Safe checks first, then a pro
34.1 Ignition fault, burners did not light in 4 trials Safe checks first, then a pro
34.2 Flame lost before the heating blower started Safe checks first, then a pro

See all overheat codes on gas furnaces across brands.