Furnace stops and displays E04, often after running for a short time; may relate to overheating.

Safety: Turn off power and allow the furnace to cool before touching any pipework.

• Check system pressure: Verify that the heating circuit pressure is within the recommended range. If low, top up via the filling loop as per the user manual.
• Bleed radiators and air vents: Air in the system can cause poor circulation and overheating. Use a radiator key to bleed radiators starting from the lowest floor upwards. Recheck pressure afterward.
• Verify pump operation: When calling for heat, place a hand on the circulation pump (inside the furnace or external, depending on model). You should feel vibration and slight warmth. If the pump is silent or extremely hot, it may be seized.
• Check flow and return valves: Ensure isolation valves on the flow and return pipes under the furnace are fully open. Also check that motorized zone valves (if fitted) are not stuck closed.
• Inspect flow sensor wiring: Locate the flow NTC sensor on the flow pipe or heat exchanger. Check that its connector is firmly seated and that the cable is not damaged.
• Reset and test: After restoring circulation and pressure, reset the furnace. If E04 recurs, the primary heat exchanger may be scaled or the pump may need replacement by a professional.