Boiler starts then shuts down after a few seconds without showing a specific error code

Safety: Repeated short cycling can damage components. Do not ignore this behaviour.

• Check radiator valves: Ensure enough radiators are open to absorb the boiler output.
• Verify system pressure and bleed air: Poor circulation can cause rapid temperature rise and shutdown.
• Observe flue terminal: Check for signs of poor combustion (sooting, staining) and call a technician if seen.
• Technician tasks: A professional can adjust boiler parameters (minimum output, pump overrun), check flame detection, and recommend system changes (buffer tank, additional radiators) if the boiler is oversized.