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

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

Steps to check:

• Air filter and registers: Replace dirty filters and ensure all supply and return registers are open.
• Blower speed: Listen for weak airflow. If airflow appears low, the blower motor or capacitor may be failing and requires professional testing.
• Inspect limit wiring: With power off, check that all limit switch wires are firmly connected and show no signs of burning or brittleness.
• Reset lockout: Turn off power for 30–60 seconds to clear the lockout. If the code returns, discontinue using the furnace until it has been serviced.

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