Furnace LED shows 13 flashes; unit may not start or may shut down unexpectedly.

Safety: Overheating can damage the furnace. Do not repeatedly reset power without addressing airflow issues.

Steps to check:

• Air filter and registers: Replace dirty filters and open all supply and return registers.
• Blower speed: Listen for weak airflow. If airflow seems low, the blower motor or capacitor may be failing and needs professional testing.
• Inspect limit wiring: With power off, check that all limit switch wires are firmly connected and not burned or brittle.
• Reset lockout: Turn off power for 30–60 seconds to clear the lockout. If the code returns, stop using the furnace until serviced.

When to call a pro: A technician should measure temperature rise, inspect the heat exchanger and duct system, and test/replace limit switches as needed.