Boiler stops with A03, temperature rises quickly, radiators very hot, pump may be noisy

Safety: Turn off the boiler and allow it to cool completely before touching any pipes. Do not remove any covers that require tools.

User checks:

• System pressure: Check the pressure gauge on the Fondital boiler. It should typically be around 1.0–1.5 bar when cold. If below 1 bar, top up via the filling loop as per the user manual, then bleed radiators and recheck pressure.
• Radiator valves: Ensure key radiators have valves open to allow circulation.
• Bleed air: Bleed air from radiators and any accessible manual air vents. Trapped air can cause overheating.
• Reset: After cooling and correcting pressure/air, reset the boiler once and observe operation.

Technician actions:

• Check operation of the circulation pump (Fondital OEM pump or Grundfos/Wilo equivalent), freeing or replacing if seized.
• Inspect and chemically clean or replace the primary heat exchanger if scaled or blocked.
• Test the overheat thermostat and flow/return NTC sensors for correct resistance values.
• Verify correct wiring and operation of the pump relay on the main PCB.