Carrier 59SC5A Error Code 13: Limit Circuit Lockout

What Does Code 13 Mean?

Status code 13 on the Carrier 59SC5A is a limit circuit lockout. The HK42FZ036 board enters it when a limit or flame rollout switch has been open for longer than 3 minutes, shutting the furnace down as a hard safety stop to protect the heat exchanger. The control will auto-reset after three hours.

Code 13 is the escalated form of code 33 (limit circuit fault). Code 33 flashes while a limit or rollout switch is open and the blower runs to cool the furnace; if that open condition clears within 3 minutes, operation resumes, but if it lasts longer than 3 minutes the board escalates to this lockout, code 13. So a recurring 13 usually means the underlying overheating condition behind a 33 was never resolved.

The most common cause is reduced airflow across the heat exchanger from a dirty or clogged air filter, followed by blocked or closed supply and return vents. When airflow drops, the heat exchanger overheats and the limit switch opens. A defective blower motor or capacitor can produce the same overheating but is uncommon and requires professional service. Note that a tripped flame rollout switch (which points to a combustion or venting problem, not just airflow) latches and needs a manual reset by a technician.

What You'll Notice

• The furnace is completely shut down with no heat, and code 13 is flashing
• The air filter is visibly dirty or overdue for replacement
• Some supply registers or return grilles are closed or blocked by furniture, rugs, or drapes
• The furnace may have been short-cycling on heat (code 33) before it locked out
• The furnace stays off for up to three hours before attempting to restart on its own

Common Causes

How This Is Diagnosed

The cause is isolated from the most common and safest outward. Airflow is checked first: the filter, then that every supply and return register is open, since restricted airflow is the leading reason the limit opens. If airflow is good and the code persists, a technician looks at the blower itself (a loose wheel, dust-loaded blades, or a weak motor/capacitor moving too little air) and confirms whether the trip is the temperature limit or the flame rollout switch. A tripped rollout switch is treated as a combustion or venting problem and is not reset until that root cause is found.

How to Fix It: Replace the Air Filter and Restore Airflow

What You'll Need

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

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF, and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Check and replace the air filter Remove the filter from the return duct or furnace filter slot. If it is dirty, clogged, or older than about three months, replace it with a new filter of the correct size. Make sure the airflow arrow points toward the furnace.
3. Open all supply and return vents Walk through every room and confirm all supply registers and return grilles are fully open and not blocked by furniture, rugs, or curtains. Even a few closed vents can restrict airflow enough to overheat the heat exchanger.
4. Inspect the blower area With the power still off, look at the blower wheel through the access opening. Note any heavy dust buildup on the blades or a wheel that looks loose or wobbly, and stop here for a professional if you see either. Do not attempt to remove or service the blower yourself.
5. Restore power and gas, then test Turn the gas valve back to ON and the breaker to ON. You can clear the lockout by cycling power (off 30 seconds, then on) instead of waiting the full three hours. 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 replacing the filter and opening all vents
• The flame rollout switch has tripped (it latches and requires a manual reset tied to finding a combustion or venting fault)
• The blower wheel is loose, wobbling, damaged, or heavily caked with dust
• The blower motor runs slowly, makes unusual noises, or does not start
• The furnace keeps locking out even with a clean filter and open vents

✓ Verified against manufacturer service manual — March 2026