Furnace shows E13; blower runs but no heat, often after a long run cycle.

Safety: Turn off power and allow the furnace to cool before touching any limit switches.

• Check air filter and vents: Replace dirty filters and ensure all registers are open.
• Inspect vent pipe: Look for sagging sections that may trap condensate, or long horizontal runs without proper slope. Correct obvious issues if accessible.
• Locate auxiliary limit: Find the small disc-type limit switch on or near the flue collector or secondary heat exchanger. Check for loose wires or signs of overheating.
• Reset (if manual type): If the switch has a reset button, press it once. If it trips again, stop using the furnace and call a technician.

Persistent E13 suggests a venting or heat exchanger restriction that requires professional cleaning and inspection.