Heater shows E13 and shuts down; may occur after long run times or in hot equipment rooms.

Safety first: Turn off power and gas. High flue temperatures can be a fire hazard; do not continue operating the heater until the cause is found.

Step-by-step checks:

• Inspect flue system: Check for obstructions, crushed sections, or excessive elbows in the flue. Ensure the flue diameter and length comply with Pahlen installation guidelines.
• Check combustion air: Verify that ventilation openings to the equipment room are not blocked. For outdoor units, ensure louvers and grilles are clear.
• Inspect flue gas sensor: Locate the flue gas temperature sensor on the flue collector. Ensure it is firmly attached and its wiring is intact. Test its resistance against the Pahlen chart.
• Evaluate heat exchanger condition: If the heater is older or has seen poor water chemistry, internal scaling may be causing excessive flue temperatures. This typically requires professional descaling or replacement.

When to stop: Flue and combustion issues should be evaluated by a qualified gas technician to ensure safe operation.