E03 error and furnace stops; display may indicate overheat or high boiler temperature

Critical safety: Do not attempt to reset an overheated Defro furnace until the boiler temperature has dropped below 40°C. Risk of scalding and system damage.

• Check the system pressure and verify that all flow and return valves to the heating circuit are open.
• Listen/feel for operation of the circulation pump (often a Grundfos/Wilo‑type pump mounted on the flow line). If it is silent and not vibrating, it may be seized or not powered.
• Ensure all radiators or underfloor heating loops are not shut off simultaneously, which can stop flow.
• Locate the manual reset safety thermostat on the boiler body (usually a red button). Once the boiler has cooled, press the reset button firmly until it clicks.
• Inspect the boiler temperature sensor (NTC) inserted into the boiler pocket. Check wiring for damage or disconnection.
• Bleed air from the system using radiator or air vent valves if some radiators are cold and pipes gurgle.

Do not bypass safety devices. If the overheat thermostat trips repeatedly, contact a qualified technician to check pump performance, system design, and control settings.