Boiler displays E9 and shuts down, often accompanied by rapid temperature rise or banging noises before lockout.

Safety first: Isolate electrical power and allow the boiler to cool fully. Do not remove the combustion cover unless you are a qualified engineer.

• Check system pressure: On the Worcester Bosch pressure gauge, ensure pressure is around 1.0–1.5 bar when cold. If below 1.0 bar, repressurise via the filling loop according to the user manual.
• Bleed radiators: Air in the system can cause localised overheating. Use a radiator key to bleed air from the highest radiators, topping up pressure as needed.
• Verify pump operation (basic check): With power restored, call for heat and lightly touch the pump body (usually a Grundfos or Wilo pump inside the boiler). You should feel vibration and slight warmth. If silent and cold, the pump may be seized or not powered.
• Clean system filter: If a Worcester Bosch system filter or magnetic filter is installed on the return pipe, isolate it, depressurise, and clean out sludge and debris.
• Engineer checks: A Gas Safe engineer should check the primary heat exchanger for blockage, verify pump speed and operation, test the overheat thermostat and NTC sensors, and flush or chemically clean the system if sludge is present.