Furnace stops and displays 04, often after overheating, radiators very hot, burner off

Safety: Turn off power and allow the unit to cool before touching any internal components. Hot water and surfaces can cause burns.

User-level checks:

• 1. Check system pressure: On the Immergas display or 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 and bleed radiators.
• 2. Open valves: Ensure all radiator valves and any external isolation valves on the flow/return pipes are open. Closed valves can stop circulation and cause overheating.
• 3. Bleed air: Bleed radiators starting from the highest floor to remove trapped air that can block flow. Re-pressurize the system as needed.

Technical checks (technician):

• Verify operation of the circulation pump (check for vibration and current draw). Replace the pump if seized or weak.
• Inspect and clean the primary heat exchanger if scaled or sludged.
• Check the safety thermostat (manual reset type on some models) and wiring.

After resolving the cause, reset the fault via the control panel. If 04 recurs frequently, a professional hydraulic and combustion check is required.