The short answer
Code 33 on a Carrier furnace in Northern Virginia means one of the switches that watch for overheating or flame in the wrong place is open: the limit switch, a flame rollout switch or the blocked vent safeguard switch. If it stays open more than 3 minutes, the furnace locks out and the code becomes 13. Carrier’s checklist starts with airflow, because a clogged filter or a dusty blower wheel lets the furnace run hot enough to open the limit. A rollout trip is a different story, and it needs a technician before anyone resets it twice.
What you’ll notice
- The burners light, run a few minutes, then shut off while the blower keeps going. Carrier’s board runs the blower for 4 minutes to cool things down.
- Warm air in bursts, then cool air, over and over.
- 3 short and 3 long flashes for 33, or 1 short and 3 long for 13.
- With a 13, the furnace stops trying. It clears itself after 3 hours or a power cut.
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How to read the code
Look through the small window in the lower door. Short flashes are the first digit and long flashes are the second, so 33 is 3 short and 3 long, then a pause before it repeats. A steady light is normal, and rapid flashing means the line voltage polarity is reversed. On older boards with a red light, cutting the power erases the code, so count it before you flip the switch. Newer Carrier boards keep the last 7 faults through a power cut and print a code with a third digit, like 13.1, on a small display. Our full list of Carrier furnace codes has every one.
Which switch tripped, and why
Swipe to see all columns →
Swipe to see all columns →
| Switch | What trips it | How it resets |
|---|---|---|
| Limit switch | Too little air across the heat exchanger: a dirty filter, closed or blocked vents, a dusty or loose blower wheel, a weak blower motor or capacitor | By itself, once it cools |
| Flame rollout switch | Flame reaching outside the burner area, which points at the heat exchanger, the burners, the venting or the gas input | Only by hand, after a tech finds the cause |
| Blocked vent safeguard switch | Exhaust backing up into the furnace instead of going out the vent | Only by hand, after a tech finds the cause |
Carrier’s list for a 33 also includes a bad switch or loose connections, so a tech tests the switch itself before blaming the airflow. Rollout switches are small round disks on the burner box with a reset button on top. If one of those popped, it’s not the filter.
What you can safely check
- Pull the filter. If it’s gray and packed, put in a new one. Our filter guide covers which MERV rating won’t choke the airflow.
- Open the supply registers and clear the return grilles. Furniture on a return is enough to do it.
- Make sure the blower door is on tight.
- Count the code, turn the furnace off for 30 seconds and back on. If 33 comes back within a day, call.
What not to do
Reset a rollout switch once at most. If it trips again, leave the furnace off and call, because flame reaching where it shouldn’t scorches wiring and points at the heat exchanger or the venting. Never jump or bypass a safety switch to get heat back. Each one is there to stop the furnace from burning gas when something is wrong, and the wrong thing is often exhaust, which carries carbon monoxide. If you smell gas, don’t flip any switches: leave the house and call your gas company or 911 from outside.
What a technician does
We find out which switch opened, then measure what’s behind it: the temperature rise across the heat exchanger against the number on the rating plate, the static pressure in the ducts, the blower motor and capacitor, and the gas input. On a rollout we look at the heat exchanger and the venting before anything gets reset. A dust-packed wheel gets cleaned; our blower wheel guide shows what that looks like.
What it costs to have us look
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: furnace short cycling, a burning smell from the furnace, the full Carrier code list and our Carrier repair page.
Every code meaning here comes from Carrier’s own documents: 58SC0A installation instructions and 59MN7C manual.
Call or book
Call (703) 997-9757 and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does code 33 mean on a Carrier furnace?
A limit switch, a flame rollout switch or the blocked vent safeguard switch is open. Start with the filter and the airflow. If it’s the rollout switch, call.
What’s the difference between Carrier code 33 and code 13?
33 means a limit, rollout or vent switch is open right now. 13 means it stayed open more than 3 minutes and the furnace locked out. 13 clears itself after 3 hours or a power cut.
Can a dirty filter cause code 33?
Yes. Less air across the heat exchanger lets it run hot enough to open the limit switch, and Carrier’s checklist starts with the filter and duct restrictions.
Should I reset the rollout switch on my Carrier furnace?
Once at most. If it trips again, leave the furnace off and call, because flame is reaching where it shouldn’t.
