Boiler shows E9 and shuts down, may overheat quickly or make kettling noises before lockout.

Safety first: Turn off power at the fused spur and allow the boiler to cool fully. Do not remove the combustion cover unless qualified.

• Check system pressure: On the pressure gauge, ensure cold pressure is around 1.0–1.5 bar. If below 1.0 bar, repressurise via the filling loop according to the user manual, then reset the boiler.
• Bleed radiators: With the system cool, bleed air from radiators starting from the lowest floor. Top up pressure again afterwards.
• Verify pump operation (audibly): When the boiler calls for heat, listen for the circulation pump running. A silent pump or loud humming may indicate seizure or failure and needs an engineer.
• Check all valves: Ensure all radiator valves and any isolation valves near the boiler are open to allow circulation.
• If E9 persists: The primary heat exchanger may be scaled or sludged, or the STB sensor may be faulty. A Gas Safe engineer should check pump performance, clean/flush the system, inspect the heat exchanger, and test/replace the safety temperature limiter sensor.