Boiler displays D3; the burner stops and the boiler remains off for a defined waiting time.

Safety: No immediate safety risk; do not open sealed combustion parts.

• Wait for timer: D3 often indicates a normal waiting period. Wait the specified time (see Junkers manual) and see if the boiler restarts automatically.
• Check thermostat settings: Ensure room thermostat and TRVs are correctly set and not causing rapid on/off cycling.
• Increase system volume: If the system is very small (few radiators), consider adding a buffer tank or an additional radiator (installer task).
• Controller configuration: Have a technician check and adjust anti-cycling parameters in the Junkers controller to better match the installation.