Boiler stops and displays E04, often after running for a short time; casing may feel very hot, radiators may be partially warm.

Safety: Turn off power and allow the boiler to cool fully before touching pipework or opening the case.

• Check system pressure: Ensure the pressure gauge reads around 1.0–1.5 bar when cold. If low, top up via the filling loop as per the Atlantic manual.
• Bleed radiators: Air in the system can cause local overheating. Use a radiator key to bleed air from the highest radiators first, then recheck and adjust system pressure.
• Check pump operation: With the boiler running (if it will reset), place a hand on the circulation pump body. You should feel vibration and slight warmth. If the pump is silent and very hot, it may be seized.
• Clean system filter: If your Atlantic installation includes a magnetic or mesh filter on the return line, isolate it and clean out sludge to improve flow.
• Call a technician: If E04 persists, a professional should check the overheat thermostat, NTC sensors, pump performance, and inspect the primary heat exchanger for blockage.