Display shows E04, boiler shuts down and will not restart until reset.

Safety: Allow the boiler to cool fully before touching any pipework. Overheat conditions can cause scalding.

• Check system pressure: Verify the pressure gauge on the boiler is within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the Thermona filling loop following the user manual.
• Bleed radiators: Air in the system can cause local overheating. Bleed radiators starting from the lowest floor, then recheck and adjust system pressure.
• Ensure valves are open: Confirm all isolation valves under the boiler and on the heating circuit are fully open.
• Pump operation: When there is a heat demand, place a hand on the pump body (if accessible) to feel for vibration. If the pump is silent and cold, it may be seized or not powered.
• Reset the boiler: After cooling and checks, reset the boiler. If E04 reappears quickly, the overheat thermostat or NTC sensor may be faulty or the heat exchanger may be blocked.
• Engineer tasks: A Thermona technician should check the primary heat exchanger for scale or sludge, verify pump performance, and test the overheat thermostat and flow NTC sensor with a multimeter.