If your Carrier furnace is flashing a 33 on the little LED behind the blower door, it's telling you the limit circuit tripped — the furnace got too hot inside and shut the burners down to protect itself. On Carrier, Bryant, and Payne furnaces (they share the same control board), code 33 is one of the most common — and most often DIY-fixable — codes there is, because the usual cause is simply restricted airflow.
Reading the code: Carrier boards flash a 2-digit code — short flashes count the tens, long flashes count the ones. Three short + three long = 33. Code meanings can vary by control-board generation, so always confirm against the diagnostic label printed inside your furnace's blower compartment.
What code 33 means
Code 33 is a limit-circuit fault: a single, soft trip of the high-temperature limit switch. That switch watches the temperature around the heat exchanger; when air isn't moving fast enough to carry the heat away, the furnace overheats and the limit opens to shut the gas off. The furnace usually tries again on its own.
It sits in a family of related Carrier codes:
- 33 — limit circuit fault (a soft, single trip). This page.
- 13 — limit circuit lockout (the limit or rollout switch stayed open too long, and the furnace has locked out). This is where an ignored 33 ends up.
- 31 — pressure switch didn't close (venting/inducer problem).
- 34 — ignition proving fault (flame not sensed).
The important takeaway: 33 is a warning about airflow. Fix the airflow and it usually goes away. Ignore it and it escalates to a 13 hard lockout and no heat at all.
Common causes
When a Carrier throws 33, here's what's behind it, most common first:
- A dirty air filter. By far the #1 cause. A clogged filter starves the blower of return air, the furnace overheats, and the limit trips. The cheapest fix in HVAC.
- Closed, blocked, or too few supply/return vents. Furniture over a return, too many closed registers, or a shut damper chokes airflow.
- A dirty blower wheel or failing blower motor. If the blower can't move enough air even with a clean filter, the furnace overheats.
- A restricted or undersized duct system. Crushed flex duct or an undersized return keeps tripping the limit.
- A failing high-limit switch. Less common — the switch itself weakens and trips early even when airflow is fine.
The usual fix
Our tech confirms the furnace is actually overheating versus a bad switch: check and replace the filter, inspect the blower wheel and motor and measure the temperature rise across the furnace, verify ducts and registers are open and flowing, and test the limit switch itself. Most code-33 calls end with restored airflow (filter/blower/duct) rather than a part; when the limit switch or blower motor has genuinely failed, we replace it and confirm the furnace runs a full heat cycle without tripping.
DIY vs. calling a pro
Safe to try yourself first (free):
- Turn the furnace off and change the air filter if it's dirty — then give it 20–30 minutes and try again. This alone clears a large share of code-33 calls.
- Open blocked vents and returns. Make sure furniture, rugs, or closed registers aren't choking airflow, and open any closed dampers.
- Power-cycle at the switch or breaker to clear the code after fixing airflow.
Call a pro when:
- Code 33 comes right back after a fresh filter and open vents — the restriction is deeper (blower, ducts, or the switch).
- It's escalated to a 13 lockout and you have no heat.
- The blower runs weak, cycles oddly, or you smell a hot/burning odor.
The limit switch and blower work involve line voltage and the gas-fired heat exchanger — and a furnace that keeps overheating can crack the heat exchanger (a carbon-monoxide risk), so persistent 33s are worth a tech's eyes.
What it costs in the Bay Area
The good news: most code-33 fixes are inexpensive, because the cause is usually a filter or airflow. When a part is needed, a high-limit switch is a modest repair and a blower motor runs about $400–$700 in our pricing depending on the model. We charge a diagnostic fee that's credited toward the repair when you approve the work, and you always get a firm, upfront price first.
Book Carrier furnace service
Triton services Carrier, Bryant, and Payne furnaces — plus every major brand — across the SF Bay Area & Peninsula, with same-week and often same-day appointments. If your Carrier is stuck on code 33 (or has locked out on 13), we'll get the airflow and heat back.
Call (888) 993-5699 or book online for same-day furnace service.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Carrier Furnace Code 13 (Limit Lockout) — where an ignored 33 ends up
- Carrier Furnace Code 31 (Pressure Switch) — the venting code
- Furnace Not Heating / No Heat — the symptom behind a limit trip
- Furnace Short Cycling (On and Off) — overheating that cycles
- Carrier / Bryant HVAC Repair — brand service hub
Frequently asked questions
What does code 33 mean on my Carrier furnace? Code 33 is a limit-circuit fault — the high-temperature limit switch tripped because the furnace overheated, almost always from restricted airflow. It's usually a soft, single trip, and the most common cause by far is a dirty air filter or blocked vents. Carrier, Bryant, and Payne furnaces share this code.
How do I fix a Carrier code 33 myself? Turn the furnace off, replace a dirty filter, and make sure supply and return vents aren't blocked by furniture or closed registers. Wait 20–30 minutes and restart. If the code clears and the furnace heats normally, you're done. If 33 comes right back, the restriction is deeper — blower, ducts, or the limit switch — and needs a tech.
Is Carrier code 33 the same as code 13? They're related. Code 33 is a single soft limit trip (a warning), while code 13 is a limit-circuit lockout — the limit or rollout stayed open too long and the furnace locked out with no heat. An ignored 33 commonly escalates into a 13, so it's worth fixing the airflow early.
Can a dirty filter cause a Carrier furnace to flash 33? Yes — a clogged filter is the single most common cause. It starves the blower of return air, the furnace overheats, and the limit switch trips to shut the gas off. Changing the filter is the first thing to try and it's the cheapest fix in HVAC.
Is a furnace code 33 dangerous? A one-time 33 that clears after a filter change isn't an emergency, but a furnace that keeps overheating and tripping the limit can eventually crack the heat exchanger, which is a carbon-monoxide risk. If 33 keeps returning, turn the furnace off and have it inspected.
How much does it cost to fix a Carrier code 33 in the Bay Area? Most 33s are cheap because the cause is airflow — often just a filter. If a part has failed, a limit switch is a modest repair and a blower motor runs about $400–$700 in our pricing. We credit the diagnostic fee toward the repair and quote a firm price before any work.