Heater intermittently shuts down with F13, especially in very cold or hot ambient conditions.

Safety: Turn off power and gas. Flue components can be hot; allow to cool.

• Inspect flue terminal: Outside, ensure the flue outlet is clear of obstructions and meets clearance requirements.
• Check flue sensor: Inside the casing, locate the flue gas temperature sensor clipped to the flue or combustion chamber outlet. Confirm it is firmly attached and wired correctly.
• Look for discoloration: A sensor that is darkened or cracked may have overheated and should be replaced by a professional.
• Reset and monitor: After reseating, reset the heater. If F13 recurs, do not continue to use the appliance; have the flue and combustion checked by a qualified engineer.