Boiler shows E9 and shuts down, may attempt to start then quickly lock out, flow temperature may rise rapidly.

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

User-level checks:

• Check system pressure is 1.0–1.5 bar when cold. If low, top up via the filling loop and bleed radiators to remove air.
• Ensure all radiator and isolation valves under the boiler are open to allow circulation.
• Press the reset button once after the boiler has cooled.

Engineer checks:

• Verify operation of the circulating pump (Wilo/Grundfos unit used in Worcester Bosch boilers): check for vibration, electrical supply, and free rotation of the rotor. Free a stuck rotor or replace the pump head if seized.
• Bleed air from the pump and high points in the system using the automatic air vent and manual vents.
• Inspect the primary heat exchanger for scale or sludge blockage; powerflush the system and chemically clean or replace the heat exchanger if restricted.
• Test the flow NTC sensor for correct resistance at given temperatures; replace if out of specification.
• Check the overheat thermostat / safety temperature limiter for correct reset and continuity; replace if faulty.