Display shows E04, boiler stops heating, temperature on display may be very high.

Safety: Turn off the boiler and allow it to cool completely. Do not bypass safety devices.

• Check system pressure: On the Klimosz pressure gauge, ensure pressure is within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the filling loop according to the manual.
• Bleed radiators and boiler: Use bleed valves on radiators and the boiler’s automatic air vent to remove trapped air that can cause local overheating.
• Verify pump operation: When calling for heat, listen for the circulation pump. If it is silent or very hot to the touch, it may be seized. Gently turn the pump shaft using the service screw (if provided) to free it.
• Inspect valves: Ensure all isolation valves under the Klimosz boiler and on the heating circuit are open. Closed valves can cause rapid overheating.
• Reset safety thermostat: Some Klimosz models have a manual reset overheat thermostat on the heat exchanger. After cooling, press the reset button if accessible.
• Restart: Turn the boiler back on. If E04 recurs quickly, the pump, NTC sensor, or heat exchanger may require professional service.