Boiler displays error 04 and will not start, often accompanied by overheat light or very hot casing.

Safety: Turn off power and allow the boiler to cool fully before touching any pipework or internal components.

• Check system pressure: On sealed Biasi systems, ensure the pressure gauge reads around 1.0–1.5 bar when cold. If low, top up via the filling loop following the manual.
• Bleed radiators: Air in the system can cause local overheating. Bleed all radiators and any manual air vents near the boiler.
• Check pump operation: When there is a heat demand, place a hand on the circulation pump. You should feel vibration and slight warmth. If it is silent and cold, it may be seized or not powered.
• Reset overheat stat: Some Biasi models have a manual reset overheat thermostat on the primary heat exchanger. Once cooled, press the reset button if accessible as per the manual.
• Engineer inspection: If the fault recurs, a technician should check for sludge-blocked heat exchanger, verify pump performance, and test the NTC sensors and overheat stat.