Boiler shows E03 and shuts down; may occur shortly after ignition or during operation.

Safety: Allow the boiler to cool fully before touching any pipework. Isolate electrical power before removing covers.

• System pressure: Check the pressure gauge on the front of the Alpha boiler. It should typically be around 1.0–1.5 bar when cold. If below 1.0 bar, top up via the filling loop according to the manual, then reset the boiler.
• 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: Listen for the circulation pump running when there is a heat demand. If it is silent or excessively noisy, the pump may be seized or air-locked. A qualified person can vent the pump and check for free rotation.
• Clean system filter: If an Alpha magnetic system filter is installed on the return pipe, isolate it and clean out sludge to improve circulation.
• Reset overheat thermostat: Some Alpha models have a manual reset overheat stat inside the casing. A qualified engineer can press the reset button once the boiler has cooled and investigate the root cause.

Repeated E03 faults indicate a circulation or heat exchanger problem and should be investigated by a professional.