Boiler locks out with E02, overheats then shuts down, may make kettling noises before stopping

Safety first: Switch off power at the fused spur and allow the boiler to cool completely before touching any pipework or internal components.

• Check system pressure: On the Sime pressure gauge, ensure pressure is around 1.0–1.5 bar when cold. If below 1.0 bar, top up via the filling loop according to the manual, then reset the boiler.
• Bleed radiators: Air in the system can cause localised overheating. Use a radiator key to bleed radiators starting from the lowest floor, topping up pressure as needed.
• Verify all valves are open: Ensure the flow and return isolation valves under the boiler and any zone valves are fully open.
• Check pump operation (engineer task): With power restored, feel the pump on the return pipe inside the boiler. It should vibrate slightly and get warm. If it is silent or excessively hot, it may be seized. The engineer can remove the pump head and check for debris or replace the pump.
• Inspect primary heat exchanger (engineer task): Look for signs of scale or sludge blockage. On older Sime models, kettling noises often indicate limescale. Power-flushing the system and/or replacing the primary heat exchanger may be required.
• Test overheat thermostat and NTC sensor (engineer task): Use a multimeter to check continuity and resistance against Sime specifications. Replace faulty sensors.