Carrier furnace Code 13 (1 short, 3 long LED blinks) indicates the limit device circuit has opened due to overheating. The most common cause is a dirty air filter blocking airflow, which causes heat to build up in the heat exchanger. Check and replace the air filter first.
Look, this code isn't always a simple filter swap. I've shown up to Code 13 calls and found everything from a sock stuffed in a return vent to a heat exchanger that looked like a crumpled soda can. Ignore it long enough and you're dealing with a cracked exchanger, a potential CO leak, or a furnace that just quits on the coldest night of the year. Don't skip the flame test.
Think of Code 13 as your furnace running a fever. The high limit safety switch has opened to prevent the heat exchanger from melting or cracking. It's the most common nuisance code I deal with during the first cold snap of the year, and honestly, half the time it's a dirty filter. But before you start replacing parts, you need to verify your ductwork and filter aren't choking the system.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Clogged or dirty air filter blocking return airflow40%
Blocked supply or return air registers reducing airflow24%
High limit switch failed and trips at normal temperature14%
Blower motor not running at full speed12%
Cracked heat exchanger allowing combustion gas mixing10%
Symptoms You May Notice
Furnace runs briefly then shuts off without completing heating cycle
Control board shows 1 short blink + 3 long blinks
Blower continues to run after burners shut off (limit switch cooling mode)
House never reaches set temperature on very cold days
Visible cracks or corrosion on heat exchanger panels (requires inspection)
Can you reset a Carrier furnace to clear the 13 code?
Most Carrier models auto-reset the code once the unit cools down, but the lockout might persist. To clear it immediately, flip your furnace power switch or breaker off for 30 seconds. If you find a small red button on a switch near the burners, that is a manual rollout. Press it firmly until it clicks. If it trips again immediately, do not keep resetting it, as there is a legitimate safety risk.
Tools Required for Diagnosis
Phillips #2 screwdriverFlathead screwdriver (1/4 inch)Multimeter with continuity settingFlashlight or headlampReplacement air filter (check size printed on existing filter frame)Shop vacuum with brush attachment for blower cleaningWork gloves
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
High Limit SwitchHH12ZB193 · $15–$35
HH12ZB193
$15 – $35
Rollout Limit SwitchHH18HA499 · $12–$25
HH18HA499
$12 – $25
Frequently Asked Questions
How often should I change the air filter to prevent Carrier Code 13?
Honestly, the 3-month rule printed on the filter box is kind of a lie for most families. If you've got a shedding dog or kids running in and out, that filter can be done in 30 days. I tell my customers to check it on the first of every month, takes about 10 seconds. Hold it up to a light. If you can't see through it, swap it out. A $10 filter beats a $300 service call every time, and it's basically the cheapest insurance you can buy for that heat exchanger.
Can a cracked heat exchanger cause Code 13 on a Carrier furnace?
Absolutely it can. When that metal cracks, pressure from the blower pushes air right into the burner area and disrupts the flame. That disruption causes heat to build up in weird spots and the limit switch trips. If you're seeing Code 13 and there's also a weird chemical smell or something like dirty socks coming from your vents, shut it down immediately. That's a classic sign of combustion gases leaking through a secondary heat exchanger, which is really common on high-efficiency Carrier models that are 10 or more years old.
Is there a manual reset button for Code 13 on a Carrier furnace?
There are actually two different types of limits on most Carriers. The main high limit switch on the back wall is auto-reset, meaning it clicks back on once the furnace cools down and you don't have to touch it. But the rollout switches near the burners have a little red button that pops up when they trip. You can press it once to reset. But here's the thing: if that button pops again right away, don't keep resetting it. The furnace saw actual flames rolling out somewhere bad, and that's not something you want to ignore.
My Carrier furnace keeps showing Code 13 even with a clean filter. What else could cause it?
That's usually a static pressure problem. Your ductwork might be undersized for the furnace, or you could have a dirty evaporator coil sitting on top of the unit that's acting like a clogged filter. I also see weak blower capacitors a ton on these units. The motor spins and sounds fine, but it's running at maybe 60% speed, and that's not enough to keep the heat exchanger cool under a heavy heating load. A capacitor is usually a $15-20 fix if you're comfortable doing it yourself, or about $80-100 with a service call.
How do I test the high limit switch on a Carrier furnace with a multimeter?
Set your meter to continuity mode, the setting that beeps. With the furnace power off, pull the two wires off the limit switch terminals and touch your probes to both sides. It should beep right away. No beep means the switch is open and it's done. One thing people mess up: make sure the furnace has cooled down completely first, like 20-30 minutes with it off. If you test while it's still hot and it reads open, that might just be the switch doing its job. Let it cool and test again before you condemn it.
How much does it cost to fix Carrier Code 13?
Depends completely on what's causing it. New air filter is $10-20 and you do it yourself. Rollout switch reset costs nothing if you find the button. High limit switch replacement is about $20-40 for the part, and a tech will charge you $150-250 total with labor. Blower capacitor is $15-20 for the part or $80-150 with a tech. Now if you've got a cracked heat exchanger, that's a whole different conversation. You're looking at $800-1500 for the exchanger repair, and on a furnace that's 15 or more years old, most people just replace the whole thing at that point.