Furnace intermittently shuts down; LED shows 11 flashes on some York models.

Safety: Turn off power before accessing internal components. Overheating can damage the heat exchanger.

• Airflow check: Replace dirty filters, open all supply and return registers, and inspect accessible ductwork for restrictions or closed dampers.
• Blower speed: On some York furnaces, blower speed taps can be adjusted on the control board. Verify that heating speed is not set to the lowest tap if overheating occurs (this is best done by a technician).
• Inspect limit switches: Locate the main limit and any auxiliary limits on the blower housing or near the burners. Check for loose wiring and signs of heat damage. Manual-reset limits should be reset only once; repeated trips indicate a real problem.
• Reset lockout: Cycle power to clear the lockout. If the furnace quickly returns to an 11-flash after a heat call, stop using it and call a professional.
• Professional evaluation: Persistent limit lockouts may indicate a cracked heat exchanger or improper gas input, which require specialized testing.