Carrier 59SC2B Error Code 33: Limit Circuit Fault

What Does Code 33 Mean?

Status code 33 (three short flashes, three long flashes) on the Carrier 59SC2B is a limit-circuit fault that is happening right now. A limit switch or flame-rollout switch is open, so the burners are off while the blower runs to cool the heat exchanger. The blower runs for 4 minutes or until the open switch remakes, whichever is longer.

Code 33 is the live warning that precedes the code 13 lockout. If the switch stays open longer than 3 minutes, the board escalates to lockout 13; if it remakes within 3 minutes, code 33 keeps flashing only until the blower shuts off. One important distinction: an ordinary limit switch resets itself when it cools, but a tripped flame-rollout switch requires a manual reset by a technician and signals a real combustion or venting problem.

The most common cause is a dirty air filter or restricted duct system that starves the heat exchanger of airflow and lets it overheat. Other causes include a restricted vent, improper vent sizing, excessive wind, a loose blower wheel, a failing blower motor or capacitor, defective switches or connections, and inadequate combustion air (which trips the rollout switch). The airflow causes are homeowner-checkable; the blower and combustion causes are not.

What You'll Notice

• The blower runs (often on high) to cool the furnace while little or no heat is produced
• The furnace stops heating mid-cycle, then may resume and trip again — short cycling on the limit
• Airflow at the registers is weak, or the filter is visibly dirty
• If the condition lasts more than about 3 minutes it becomes the code 13 lockout
• Code recall shows a three-short, three-long pattern

Common Causes

How This Is Diagnosed

Airflow is checked first because it is the dominant cause: the filter, return path, and supply/return registers are inspected and corrected. If airflow is good and code 33 still occurs, a technician evaluates the blower wheel and motor/capacitor, the vent for restriction or improper sizing, and the switches and their connections. If it is specifically the flame-rollout switch that opened, the focus shifts to combustion-air supply and venting — a professional matter, since the rollout switch also needs a manual reset.

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 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, with the airflow arrow pointing toward the furnace.
3. Open every supply and return vent Confirm all supply registers and return grilles throughout the home are fully open and unobstructed by furniture, rugs, or curtains. Closed or blocked vents are a frequent cause of overheating on this furnace.
4. Look for obvious blower-area restrictions With the power still off, look through the access opening for heavy dust on the blower wheel and confirm the return-air path and any duct dampers are open. Do not remove or service the blower motor itself.
5. Restore gas and power, then test Turn the gas valve to ON and the breaker to ON. Set the thermostat to call for heat and watch a full cycle to confirm the furnace no longer trips the limit.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 33 keeps escalating to the code 13 lockout despite a clean filter and open vents
• A flame-rollout switch has tripped (it needs a technician's manual reset), or you see or smell scorching near the burners
• The blower does not run when the limit opens
• The blower wheel sounds loose or is rattling, or the motor makes unusual noise
• You suspect inadequate combustion air supply

✓ Verified against manufacturer service manual — March 2026