Boiler locks out with E03; fan may run but burner does not ignite.

Safety: Switch off power and allow the boiler to cool. Do not operate the boiler with the combustion cover removed.

• Check flue terminal: From outside, ensure the CTC concentric flue terminal is clear of obstructions (ice, leaves, plastic bags). Remove any blockage.
• Inspect air intake: On room-sealed models, check that the air intake section of the flue is not blocked or crushed.
• Check pressure tubes: Remove the front cover and locate the air pressure switch and silicone tubes. Ensure tubes are not split, kinked, or full of condensate. Clear or replace as needed.
• Test fan operation (technician): With power restored, command heat and listen for the fan. If the fan does not start or is noisy, test supply voltage at the fan connector. Replace the fan assembly if seized or electrically open.
• Air pressure switch test (technician): Using a manometer or multimeter, verify that the air pressure switch changes state when the fan runs. Replace the air pressure switch if it fails to operate within the specified pressure range.