Furnace shuts down and LED flashes code 13 (limit switch lockout). Blower may run continuously; burners will not light.

Safety first: Turn off power and gas supply before removing panels or touching internal components.

• Check air filter: Locate the return-air filter (often in the blower compartment or return grille). Remove and inspect. If dirty or clogged, replace with a properly sized filter matching the furnace specifications.
• Inspect registers and returns: Ensure all supply registers and return grilles are open and not blocked by furniture, rugs, or debris.
• Check blower operation: Restore power and call for heat. Confirm the indoor blower motor starts after the burners ignite. If the blower does not start, the furnace will overheat and trip the limit.
• Reset lockout: Turn power OFF for 30–60 seconds to reset the control board. Restore power and observe a full heat cycle.
• Inspect limit switch wiring: With power OFF, locate the primary limit switch mounted on the heat exchanger panel. Ensure its wires are firmly connected and not damaged.

If the furnace repeatedly trips the limit even with clean filters and good airflow, the limit switch or blower speed settings may be incorrect, or the heat exchanger may be restricted. Have a qualified technician test the limit switch and inspect the heat exchanger.