Carrier AC / Heat Pump Error 33 – Causes & Fix
A high-temperature safety switch opened, usually because too little air is moving through the furnace.
Safety first · Consult a professional
Keep the blower door on and let the fan run after the burners stop – that cooldown is the furnace protecting itself, and limit or rollout switches must never be bypassed or reset by hand. Should you notice a gas odor, leave the home first and call 911 or your gas company from outside without flipping switches on the way out; a sounding carbon monoxide alarm means everyone goes outside, you call 911, and the furnace stays off until it has been inspected.
- Gas appliance: no DIY repairs. If you smell gas, leave the building, do not use switches and call your gas utility’s emergency line from outside.
- Carbon monoxide is odorless. Keep a CO alarm near fuel-burning appliances; if it sounds, get everyone outside and call 911.
- Unplug the appliance or switch off the breaker before opening any panel. Mains voltage can kill.
At a glance
Severity: Moderate
- Meaning
- Limit Circuit Fault – Overheating (Code 33)
- Typical repair cost
- $10–$1,200
- DIY difficulty
- Moderate
Carrier gas furnaces protect the heat exchanger with a limit switch that opens when the air around it gets too hot, and the control immediately shuts off the gas. Code 33 – three short and three long flashes – means something in that limit circuit has opened: the main limit, the draft safeguard switch on 80% models, the flame rollout switch, or a blocked vent switch where one is installed. After the burners go out, the blower keeps running for 4 minutes or until the switch closes again, whichever takes longer, and then the furnace tries another cycle. In most houses the underlying problem is airflow: a clogged filter, closed or blocked registers and returns, undersized ductwork, a weak blower or capacitor, a dirty blower wheel or a dirty A-coil sitting on top of the furnace. Two-stage models switch to high heat only after two limit trips in a row on low heat, and if the switch stays open longer than 3 minutes the code turns into the lockout code 13.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| 80% furnaces such as the 58STA type | 33 | The limit circuit includes the main limit, the draft safeguard switch on the vent side and the flame rollout switch. |
| Condensing furnaces such as the 59SC5 type | 33 | Main limit and flame rollout switch, plus a blocked vent switch on models that use one. |
| Two-stage and variable-speed furnaces | 33 | After two back-to-back low-heat limit trips the furnace runs on high heat only; stored codes are cleared automatically after 72 hours. |
| Bryant and Payne furnaces | 33 | Same Carrier-family control board, same flash pattern and meaning. |
Symptoms
- Status LED shows 3 short flashes followed by 3 long ones
- Burners light and heat for a few minutes, then go out while the blower keeps running
- House never reaches the thermostat setting and the furnace short-cycles all day
- Very hot air at the registers at first, then lukewarm, with weak airflow overall
- Hot, dusty smell around the furnace cabinet
- Code later changes to 13 and the furnace stops trying
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Dirty filter, or a filter that is too restrictive for the system
- Closed or blocked supply registers and return grilles, or crushed and undersized ducts
- Weak blower motor or failing blower capacitor
- Dirty or loose blower wheel
- Dirty A-coil above the furnace restricting airflow
- Restricted vent, excessive wind or too little combustion air opening the draft safeguard or rollout switch
- Defective limit switch or a loose connection in the limit circuit
Fix it step by step
Time one complete heating cycle
After confirming the 3-short, 3-long pattern through the sight glass, raise the thermostat and time how long the burners stay lit. If they go out after a few minutes while the blower keeps pushing air for several more, you are watching the limit trip and reclose in real time.
Tools: Flashlight
Pull and inspect the filter
With the furnace service switch off, slide the filter out of its slot next to the furnace, inside the cabinet, or behind a return grille. A gray, matted filter goes straight in the trash; replace it with the same size, and be aware that very dense 1-inch pleated filters can choke the airflow on some systems.
Tools: Flashlight
Open the house up to airflow
Walk every room and open all supply registers, including those in guest rooms or the basement that someone closed to save energy. Move rugs, beds and sofas away from return grilles – a single large central return that is half covered can be enough to trip the limit.
Check the blower door and feel the registers
Make sure the blower compartment door is seated properly so the door switch is closed and the cabinet isn't leaking air. With a clean filter and all registers open, strong airflow should be easy to feel at the nearest supply; weak airflow now points to the blower motor, its capacitor, a dirty blower wheel or the coil.
Think about what changed
A new, denser filter type, duct work during a renovation, a new air conditioner coil or rooms that were recently closed off often line up with the first appearance of 33. Tell the technician about any of these, because they point to airflow rather than to the switch.
How to reset 33
- Once the limit recloses, the furnace retries automatically – a manual reset isn't always needed
- After replacing the filter or opening registers, turn the thermostat down and switch the service switch off for 30 seconds
- Restore power, raise the thermostat and watch at least one full cycle to confirm the burners keep running
Where the parts are
- 1Fan
- 2Temperature sensor
- 3CAN bus
- 4Outdoor unit
- 5Compressor
- 6Engine computer (ECM)
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| New furnace filter | Yes | $10–$40 |
| Technician diagnostic visit | No | $100–$200 |
| Blower motor capacitor replacement | No | $120–$300 |
| Limit switch replacement | No | $150–$350 |
| Blower wheel and coil cleaning | No | $150–$450 |
| Blower motor replacement (ECM motors at the top of the range or above) | No | $450–$1,200 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Book an HVAC technician when 33 keeps returning with a fresh filter and every register open, or when airflow at the registers stays weak. A technician will measure the temperature rise across the furnace against the range on its rating plate, check the blower and capacitor, and look at the coil and duct sizing – replacing the limit switch without that work usually just brings the code back.
Frequently asked questions
Is it normal that the blower keeps running after the flame goes out?
Yes. With code 33 the control keeps the blower on for at least 4 minutes, or until the limit switch closes again, to pull heat out of the heat exchanger. It is a protective cooldown, not a separate fault.
Can a high-MERV filter cause code 33?
It can. A dense 1-inch pleated filter adds a lot of resistance, and on a system with tight ductwork that alone can push the furnace over its limit. If the code began after a filter change, try a less restrictive filter of the same size or ask the technician about a deeper media cabinet.
Why did code 33 turn into code 13?
If the limit circuit stays open longer than 3 minutes, the control stops cycling and locks out with 13. That lockout means the overheating wasn't brief, so it deserves a closer look.
Will closing vents in unused rooms save money?
On a forced-air furnace it often does the opposite. Closing too many supply registers raises duct pressure, reduces airflow across the heat exchanger and can trip the limit, which is a common way homeowners trigger 33 without realizing it.
Sources
- Official Carrier user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy