Boiler stops with A03, burner off, overheat symbol may be lit

Safety: Turn off power and allow the boiler to cool completely before touching any hydraulic components. Hot water can cause severe burns.

Checks:

• Verify system pressure: On the De Dietrich display or manometer, ensure pressure is typically between 1.0–1.5 bar when cold (check your model manual). If low, top up via the filling loop according to the instructions, then bleed radiators.
• Bleed air from radiators: Use a radiator key to release trapped air starting from the lowest radiators. Keep an eye on system pressure and re‑pressurise as needed.
• Check circulation pump: Listen for the pump running when there is a heat demand. On some De Dietrich models, you can access the pump front screw and gently turn the shaft with a screwdriver to free it if stuck (power off first).
• Inspect valves: Ensure all isolation valves under the boiler and on the heating circuit are open. Partially closed valves can cause local overheating.
• Reset overheat thermostat: Some models have a manual reset button on the safety thermostat mounted on the heat exchanger. Once cooled, press the button until it clicks. If it trips again, do not continue resetting.

Call a technician if: A03 recurs, the pump does not run, or the heat exchanger is suspected to be scaled or blocked. Professional descaling or component replacement may be required.