If your Carrier furnace is flashing 13 on the little LED behind the blower door and blowing no heat, it has locked itself out on the limit circuit. On Carrier, Bryant, and Payne furnaces — which share the same control board — code 13 means the high-temperature limit switch or a flame-rollout switch stayed open too long, so the board gave up and shut the furnace down. Where a 33 is a single, soft trip the furnace recovers from on its own, code 13 is the hard lockout that a repeated or ignored 33 turns into.
Reading the code: Carrier boards flash a 2-digit code — short flashes count the tens, long flashes count the ones. One short + three long = 13. Flash-code meanings can vary by control-board generation, so always confirm the number against the diagnostic label inside your unit — printed inside the blower compartment — before you buy a part.
What code 13 means
Code 13 is a limit-circuit lockout. The high-temperature limit switch watches the heat around the heat exchanger, and a rollout switch watches for flame escaping the burner box. If either one opens and stays open for longer than about three minutes, the board stops trying and hard-locks the furnace out — no more ignition attempts until the fault is cleared. That is why a 13 leaves you with no heat at all.
The key is its relationship to code 33. A 33 is the warning — a single soft limit trip, usually from airflow that recovered on its own. When the restriction never gets fixed, or the switch itself has failed, the furnace keeps overheating until the limit holds open long enough to lock out on 13. Treat a 13 as a 33 that was left too long.
The code 13 family
| Code | Meaning | Typical cause |
|---|---|---|
| 13 | Limit circuit lockout — hard lockout, no heat | Limit or rollout switch held open too long from sustained restricted airflow, a failed blower, or a bad switch |
| 33 | Limit circuit fault — single soft trip | A brief overheat, usually a dirty filter or blocked vents; the warning that precedes a 13 |
| 31 | Pressure switch did not close | Venting or inducer restriction, blocked flue, or a condensate backup |
| 34 | Ignition proving fault — flame not sensed | Dirty or misaligned flame sensor, weak flame, or low gas pressure |
Because these boards are shared across Carrier, Bryant, and Payne, the same chart applies to all three — but read the exact pattern off your own unit's label, since it can differ by board generation.
Common causes
When a Carrier locks out on 13, here's what's usually behind it, most common first:
- Sustained restricted airflow. A very dirty filter, blocked or closed returns, or crushed/undersized ducts, so the furnace overheats cycle after cycle until the limit holds open. This is by far the most common cause.
- A failed or failing blower motor. If the blower can't move enough air even with a clean filter, the heat exchanger overheats every run and eventually locks out.
- A stuck-open or failed limit or rollout switch. The switch weakens or sticks and reports open even when airflow is fine.
- A cracked heat exchanger (chronic cases). A furnace that has overheated for a long time can crack the heat exchanger, which can trip rollout — and is a carbon-monoxide risk.
The usual fix
Our tech first confirms whether the furnace is genuinely overheating or a switch has failed: check and replace the filter, inspect the blower wheel and motor and measure the temperature rise, verify ducts and registers are open, and test the limit and rollout switches and their wiring. On a furnace that keeps locking out, we inspect the heat exchanger and combustion path before returning it to service. The usual outcome is restored airflow rather than a part; when the blower motor or a switch has truly failed, we replace it and confirm a full heat cycle with no re-lockout.
DIY vs. calling a pro
Safe to try yourself first (free):
- Clear the lockout once. Turn the furnace off at the switch or breaker for a minute, then back on, to reset the lockout a single time.
- Change a dirty filter and open blocked vents and returns before you restart — remove anything sitting over a return and open any closed registers or dampers.
- Give it 20–30 minutes and watch whether it completes a full heat cycle.
Call a pro when:
- The furnace re-locks on 13 after a fresh filter and open vents — the restriction is deeper (blower, ducts, or a failed limit/rollout switch) and repeatedly resetting a lockout only masks it.
- You smell a hot or burning odor, or the burners look like flame is rolling out of the box — that points to a heat-exchanger or combustion problem and a carbon-monoxide risk.
- The blower runs weak or won't start.
Gas, line-voltage electrical, and any refrigerant or compressor work belong with a licensed technician (EPA 608 certified for refrigerant handling). A furnace that keeps overheating and locking out can crack the heat exchanger, so a 13 that keeps coming back is worth a tech's eyes rather than another reset.
What it costs in the Bay Area
Many 13 lockouts trace back to airflow, so the fix can be as cheap as a filter and a cleared code. When a part has failed, in our pricing a blower motor runs about $400–$700 and a control board $200–$600, depending on the model. We charge a diagnostic fee that's credited toward the repair when you approve the work, with a firm, upfront price before any work begins.
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 has locked out on 13 (or keeps warning you with a 33), we'll find the airflow or switch problem and get your heat back safely.
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 33 (Limit Circuit Fault) — the soft trip that escalates into a 13
- Furnace Not Heating / No Heat — the symptom behind a lockout
- Furnace Short Cycling (On and Off) — overheating that cycles before it locks out
- Carrier / Bryant HVAC Repair — brand service hub
- Furnace Repair in San Mateo — same-day service in your city
Frequently asked questions
What does code 13 mean on my Carrier furnace? Code 13 is a limit-circuit lockout. The high-temperature limit switch or a flame-rollout switch stayed open longer than about three minutes, so the control board hard-locked the furnace out and stopped trying to ignite. It leaves you with no heat and is almost always caused by sustained restricted airflow. Carrier, Bryant, and Payne furnaces share this code.
Is Carrier code 13 the same as code 33? They are directly related. Code 33 is a single soft limit trip, a warning that the furnace briefly overheated, while code 13 is the hard lockout that a repeated or ignored 33 turns into. Fixing the airflow behind a 33 early is the way to avoid a 13 with no heat.
Can I reset a Carrier code 13 lockout myself? You can clear the lockout once by turning the furnace off at the switch or breaker for a minute, then back on, after changing a dirty filter and opening blocked vents. If it re-locks on 13, stop resetting it. The restriction is deeper and repeatedly clearing a lockout only hides a furnace that keeps overheating.
What causes a Carrier furnace to lock out on 13? The most common cause is sustained restricted airflow from a very dirty filter, blocked returns, or crushed ducts. A failing blower motor or a stuck-open limit or rollout switch can also do it, and a furnace that has overheated for a long time can crack the heat exchanger.
Is a Carrier code 13 dangerous? A single lockout that clears after a filter change is not an emergency, but a furnace that keeps overheating and locking out can crack the heat exchanger, which is a carbon-monoxide risk. If 13 keeps returning, or you smell a burning odor, turn the furnace off and have it inspected.
How much does it cost to fix a Carrier code 13 in the Bay Area? Many 13 lockouts are inexpensive because the cause is airflow, often just a filter. If a part has failed, a blower motor runs about 400 to 700 dollars and a control board 200 to 600 dollars in our pricing, depending on model. We credit the diagnostic fee toward the repair and quote a firm price before any work.