Boiler shows A05 and shuts down; may occur intermittently during operation, often related to flue or air issues.

Safety first: Turn off power and do not operate the boiler if you suspect flue blockage or fumes. Risk of carbon monoxide.

• Visual flue check (user level): From outside, ensure the flue terminal is clear of obstructions (ice, leaves, nests). Do not dismantle the flue yourself.
• Check condensate pipe: In freezing weather, ensure the external condensate pipe is not frozen or blocked. Thaw gently with warm (not boiling) water if frozen.
• Reset once: After clearing obvious obstructions, reset the boiler. If A05 returns, stop using the boiler.
• Engineer checks:
  • Test the air pressure switch tubing and connections for blockages or splits.
  • Verify correct operation of the fan and replace if not achieving required speed.
  • Inspect and clean the venturi and flue gas path.
  • Replace the air pressure switch if faulty.