Boiler shows E9 and shuts down, may overheat quickly, flow pipe extremely hot, radiators may not heat evenly.

Safety first: Switch off the boiler and allow it to cool completely. Do not remove the case unless you are competent and qualified.

Homeowner checks:

• Check system pressure is around 1.0–1.5 bar when cold; top up via the filling loop if low.
• Bleed radiators starting from the lowest floor to remove trapped air, then re-pressurise the system.
• Ensure all radiator and isolation valves near the boiler are open.
• Reset the boiler and observe operation. If E9 returns quickly, stop using the boiler.

Engineer checks:

• Test continuity of the Worcester overheat thermostat and replace if open circuit or nuisance tripping.
• Check the circulation pump is running, not seized, and that the impeller is not blocked; replace the pump head if necessary.
• Inspect the primary heat exchanger for sludge or scale blockage; powerflush or chemically clean as appropriate.
• Verify correct operation of any diverter or 3-way valve that may be restricting flow.

Persistent E9 faults indicate a serious overheating issue and should be addressed by a qualified heating engineer.