Furnace runs briefly then shuts down; LED shows code 33 (limit or flame rollout open). Blower may continue to run.

Critical safety warning: A rollout or repeated limit trip can indicate a dangerous overheating or combustion problem. Turn off gas and power if flames appear outside the burner area or if you smell strong combustion odors.

• Replace or clean filter: Install a new filter of the correct size and MERV rating. Avoid overly restrictive filters unless the duct system is designed for them.
• Check registers and dampers: Open all supply registers. Ensure manual dampers in ductwork are not closed.
• Inspect blower operation: With power on and a heat call, verify the indoor blower starts on time and runs at full speed. Listen for squealing or grinding that may indicate a failing motor or wheel.
• Reset manual rollout switches: With power off, locate rollout switches near the burners. If a manual-reset switch has tripped, press the reset button. If it trips again, shut down the furnace and call a professional.

Persistent code 33 after airflow corrections requires professional inspection of the heat exchanger, burners, and venting.