Boiler attempts to start but shuts down with E03, fan may run briefly, no flame established.

Safety first: Switch off the boiler and isolate electrical power. Do not operate the boiler if you suspect a flue gas problem; risk of carbon monoxide.

User-level checks:

• Visually inspect the outside flue terminal for obstructions (ice, leaves, nests). Remove any visible blockage without dismantling the flue.
• Check that the condensate pipe outlet is not frozen or blocked; thaw gently with warm (not boiling) water if frozen.
• Reset the boiler and observe if E03 reappears.

Technical checks (qualified personnel only):

• Inspect the entire flue system for correct assembly, slope, and obstructions; rectify any installation faults.
• Test the flue gas thermostat or fume sensor for correct operation and replace if faulty.
• Check the air pressure switch tubing for splits, blockages, or water ingress; replace tubing or switch as needed.
• Verify fan operation and speed; ensure the fan is delivering adequate airflow.
• Clean and check the condensate trap and internal condensate pathways.

Do not bypass or short any safety devices. If the cause cannot be clearly identified and rectified, leave the boiler off and call a certified technician.