Boiler locks out with A03, fan may run briefly, no ignition sequence

Safety: Turn off the boiler and do not attempt to run it with a suspected flue blockage. Risk of carbon monoxide. Ventilate the area and keep people away from the boiler if you suspect fumes.

• Check flue terminal: From outside, ensure the De Dietrich concentric flue terminal is clear of snow, plastic bags, bird nests, or other obstructions.
• Inspect condensate drain: Check the condensate pipe and trap for kinks, freezing (in cold weather), or blockages. Thaw gently with warm (not boiling) water if frozen. Clean the condensate trap if accessible per the manual.
• Check air intake: For room-sealed models, ensure the air intake is not obstructed. For open-flue models, ensure the boiler room is well ventilated and vents are not blocked.
• Technician checks: A professional should test the flue gas temperature sensor, air pressure switch, and verify correct flue installation and combustion values. Replace faulty sensors or pressure switch as needed.