Carrier Furnace Code 13: Meaning, Causes & Fixes

Last updated Jun 13, 2026 · By fixme.vip Editorial

What this code means

On most Carrier gas furnaces, Code 13 (13 flashes of the diagnostic LED) indicates a limit circuit lockout. The control board counts how many times a safety switch in the limit string — typically the high-temperature limit or a flame rollout switch — has opened during heating calls. When it opens too many times in a row, the board stops trying and locks the furnace out, flashing Code 13.

In plain terms: the furnace got too hot (or believes it did) one too many times, so it shut itself down to protect you and the equipment. This is a safety system doing its job, not a random glitch.

Important: Flash-code meanings can differ slightly between board revisions. Carrier, Bryant, and Payne share the same control platform but the legend printed inside your burner door is the authority. Always confirm Code 13 against your unit’s label before acting.

Common causes, ranked by probability

1. Restricted airflow from a dirty filter. The single most common cause. Low airflow lets the heat exchanger overheat and trips the high limit.
2. Blocked or closed supply/return registers or ducts. Furniture over vents, closed dampers, or a collapsed flex duct starve the system of return air.
3. Failing or weak blower motor / capacitor. If the blower can’t move enough air, the furnace overheats even with a clean filter.
4. Dirty blower wheel. Caked-on dust on the squirrel-cage blades drops airflow dramatically.
5. A genuinely faulty or out-of-spec limit switch. Limit switches age and can trip early or stay open.
6. Flame rollout switch tripped. This points to a more serious combustion or venting problem — a blocked flue, cracked heat exchanger, or burner issue.
7. Control board fault. Least common, but a board that misreads the limit string can lock out a healthy furnace.

Safe checks before you call anyone

These are the only checks a homeowner should do. Stop if anything looks burnt, smells of gas, or is beyond these steps.

• Check the air filter. Pull it and hold it to the light. If it’s gray and clogged, replace it — this alone clears many Code 13 events.
• Check your vents and registers. Make sure supply and return grilles are open and not blocked by rugs, furniture, or boxes.
• Check the thermostat. Confirm it’s set to Heat and the temperature is calling. Replace thermostat batteries if it uses them.
• Check the breaker. Confirm the furnace breaker hasn’t tripped. Reset it once if it has.
• One reset only. You may cycle furnace power once to clear a single lockout. If Code 13 returns, do not keep resetting it — that defeats a safety device. Call a technician.

Do not open the gas valve manually, bypass or jump out the limit/rollout switches, or remove the burner assembly.

How a technician will diagnose it

A good tech will work the airflow-and-safety chain in order, so you can sanity-check the quote:

1. Verify the code and history on the control board and ask about the symptoms.
2. Inspect the filter, blower wheel, and ductwork for restriction, and measure temperature rise across the furnace against the rating-plate spec.
3. Test the blower motor and capacitor under load to confirm it moves rated airflow.
4. Check the limit and rollout switches for continuity and trip point with a meter.
5. Inspect the heat exchanger and flue/vent if a rollout switch tripped — looking for blockage, soot, or cracks.
6. Test the control board only after airflow and switches check out.

If a tech wants to replace the board or limit switch without ever checking the filter, blower, and temperature rise, get a second opinion.

Symptom, cause and what to do

Repair costs

Honest US ranges, parts plus typical labor:

• Air filter: $10–$40 (DIY).
• Service call / diagnostic: $90–$180, often credited toward repair.
• Blower wheel cleaning: $150–$350.
• High limit switch (replace): $150–$350 installed.
• Flame rollout switch (replace): $150–$300 installed.
• Blower motor capacitor: $120–$300 installed.
• Blower motor (replace): $400–$700+ depending on PSC vs. ECM.
• Control board (replace): $400–$650 installed.
• Heat exchanger issue: often $1,000+ — frequently the trigger to weigh furnace replacement, especially on older units.

Many Code 13 calls end up being a $0–$40 filter or vent fix. Spend the most diagnostic effort there before approving parts.

Related codes

• Code 12 – Blower-on-after-power-up (board powered up while blower running); often related airflow context.
• Code 14 – Ignition lockout (failed to light), different fault chain.
• Code 33 – Limit circuit fault (a related limit-string problem on some boards).
• Code 34 – Ignition proving / flame sense failure.

Always confirm any code against the legend printed inside your furnace’s burner door, since decoding varies by board revision.