Carrier 59SC2B Error Code 13: Limit Circuit Lockout

What Does Code 13 Mean?

Status code 13 (one short flash, three long flashes) on the Carrier 59SC2B is a limit-circuit lockout. It is the escalated form of code 33: when a limit or flame-rollout switch on this single-stage furnace stays open longer than 3 minutes, the board stops trying and locks the furnace out, auto-resetting only after about 3 hours.

The limit switch is a temperature-sensing safety on the heat exchanger. When airflow across the exchanger falls too low, temperature climbs past the switch's setpoint and it opens, cutting the burners while the blower keeps running to shed heat. On the 59SC2B this protects the heat exchanger from thermal damage. Because the fault is almost always airflow-related, the fixes homeowners can safely perform target airflow.

The most common cause is a dirty or clogged air filter; closed or blocked supply and return vents are next. A less common cause is a failing blower motor or capacitor that cannot move enough air — that is not a DIY repair. Note also that code 13 can be triggered by a flame-rollout switch, which is a more serious combustion problem and requires manual reset by a technician, not a homeowner reset.

What You'll Notice

• The furnace heated for a while, then shut down and will not restart until it resets hours later
• The blower keeps running to cool the furnace even though no heat is being produced
• Airflow at the registers is weak, or the filter looks gray and clogged
• The lockout clears on its own after roughly 3 hours, then the same shutdown repeats

Common Causes

How This Is Diagnosed

The logical order is airflow first, hardware last. A clogged filter and closed or blocked vents are checked and corrected before anything else, since restricted airflow is by far the most common trigger. If airflow is confirmed good and code 13 still returns, the investigation moves to the blower motor and capacitor, the limit switch itself, and — if a flame-rollout switch is what tripped — the combustion side, which points to a venting or heat-exchanger problem that must be handled by a technician.

How to Fix It: Restore Airflow: Replace the Filter and Open the Vents

What You'll Need

• Replacement air filter (correct size for your system) 🛒 Find at FiltersFast · 🛒 Find at Amazon
• Flashlight

Steps

1. Turn off power at the breaker and shut off the gas supply valve Flip the furnace circuit breaker to OFF and turn the manual gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Check and replace the air filter Pull the filter from the return duct or furnace filter slot. If it is dirty, clogged, or more than about three months old, replace it with a new filter of the correct size, with the airflow arrow pointing toward the furnace.
3. Open every supply and return vent Walk the house and confirm all supply registers and return grilles are fully open and not blocked by furniture, rugs, or curtains. Even a handful of closed vents can restrict airflow enough to trip the limit.
4. Look for obvious airflow restrictions at the blower With the power still off, look through the access opening at the blower wheel for heavy dust buildup, and confirm the return-air path and any duct dampers are open. Do not remove or service the blower motor — that is a pro repair.
5. Restore gas and power, then test Turn the gas valve to ON and the breaker to ON. The furnace may take up to 3 hours to auto-reset, or you can cycle power once (off 30 seconds, then on) to clear the lockout. Set the thermostat to call for heat and watch a full cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 13 returns after the filter is replaced and all vents are open
• The blower does not run, runs slowly, or makes unusual noise when the limit trips
• A flame-rollout switch has tripped (it needs a technician's manual reset, not a homeowner reset)
• You see scorching or smell a hot, burning odor near the burner compartment
• The lockout recurs within a cycle or two even with a clean filter

✓ Verified against manufacturer service manual — March 2026