Carrier 59SC5A Error Code 33: Limit Circuit Fault

What Does Code 33 Mean?

Status code 33 on the Carrier 59SC5A is a limit circuit fault: a limit switch or flame rollout switch is currently open. The burners are stopped while the blower runs to cool the heat exchanger, for 4 minutes or until the open switch remakes, whichever is longer.

Timing matters here. If the switch stays open longer than 3 minutes, the board escalates to lockout code 13. If it closes within 3 minutes, code 33 keeps flashing until the blower shuts off, and normal operation can resume. In other words, code 33 is the early warning and code 13 is the hard lockout it becomes if the overheating is not relieved. A tripped flame rollout switch latches and requires a manual reset by a technician, because it signals a combustion or venting problem rather than a simple airflow issue.

The most common cause is a dirty air filter or restricted duct system reducing airflow across the heat exchanger, which drives temperatures up until the limit opens. Other causes include improper vent sizing, a restricted vent, a loose blower wheel, excessive wind, a defective blower motor or capacitor, faulty switch connections, or inadequate combustion air (which trips the flame rollout switch).

What You'll Notice

• The blower runs even though the burners are off and no heat is being produced
• The furnace short-cycles: it heats briefly, then the burners drop out while the blower keeps running
• The air filter is visibly dirty or overdue
• Some supply registers or return grilles are closed or blocked
• Repeated code 33 events sometimes end in a code 13 lockout

Common Causes

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 Flip the furnace circuit breaker 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 or clogged, replace it with a new filter of the correct size and make sure the airflow arrow points toward the furnace.
3. Open all supply and return vents Confirm every supply register and return grille is 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 heavy dust buildup on the blades or a wheel that looks loose or wobbly, and stop for a professional if you see either. Do not 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. Set the thermostat to call for heat and watch a full cycle to confirm the burners stay lit and the code does not return.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 33 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, damaged, or heavily caked with dust
• The blower motor makes unusual noises or seems to run slowly
• You suspect inadequate combustion air or a venting problem

✓ Verified against manufacturer service manual — March 2026