Carrier 59SC5A Error Code 33: Limit Circuit Fault

What Does Code 33 Mean?

Status code 33 on the Carrier 59SC5A indicates a limit circuit fault. A limit switch or flame rollout switch is currently open, which stops the burners while the blower continues running to cool the heat exchanger. The blower will run for 4 minutes or until the open switch remakes, whichever is longer.

If the switch stays open for longer than 3 minutes, the code will escalate to lockout code 13. If the switch closes within 3 minutes, code 33 will continue to flash until the blower shuts off. The flame rollout switch requires manual reset if it has tripped.

The most common cause is a dirty air filter or restricted duct system that reduces airflow across the heat exchanger, causing it to overheat. Other possible causes include improper vent sizing, a restricted vent, a loose blower wheel, excessive wind, a defective blower motor or capacitor, defective switch or connections, or inadequate combustion air supply (which would trigger the flame rollout switch).

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 Locate the furnace circuit breaker and flip it to OFF. Turn the 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 air filter from the return air duct or furnace filter slot. If it is visibly dirty or clogged, replace it with a new filter of the correct size. Make sure the airflow direction arrow on the new filter points toward the furnace.
3. Verify all supply and return vents are open Walk through every room and ensure all supply registers and return grilles are fully open and not blocked by furniture, curtains, or rugs. Even a few closed vents can significantly restrict airflow and cause overheating.
4. Inspect the blower area With the power off, look at the blower wheel through the access panel opening. Check for excessive dust buildup on the blower blades, which can reduce airflow. Also verify the blower wheel is securely attached to the motor shaft and not wobbling.
5. Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker to ON. Set your thermostat to call for heat and monitor the furnace through its heating cycle.

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 (a red button on the switch needs manual reset)
• The blower wheel is loose, damaged, or excessively dirty
• You hear unusual noises from the blower motor
• You suspect inadequate combustion air supply

✓ Verified against manufacturer service manual — March 2026