LED flashes code 33; furnace shuts down on high limit, blower may run continuously.

Safety: Overheating can damage the heat exchanger. Turn off power before removing panels. Do not bypass the limit switch.

Steps:

• Replace air filter: Locate the return air filter (in the furnace or return grille). Replace with the correct size and MERV rating. Avoid overly restrictive filters.
• Open all registers: Ensure all supply registers and return grilles are open and not blocked by furniture, rugs, or drapes.
• Check blower operation: Observe that the indoor blower starts within a minute of burner ignition and runs strongly. Listen for unusual noises or slow start.
• Inspect blower wheel: With power off, remove the blower compartment door and inspect the blower wheel for heavy dust buildup that can reduce airflow. Clean if necessary.
• Inspect limit switch wiring: Ensure the wires to the primary limit switch are secure and not damaged.

When to call a technician: If code 33 recurs despite good airflow, the blower motor, capacitor, or limit switch may be failing, or the heat exchanger may be restricted. Professional diagnosis is recommended.