F9 error, unit locks out, may occur intermittently during operation

Safety: Isolate electrical power and close the cold water inlet valve to the heater. Relieve pressure by opening a hot water tap.

• Clean inlet filter: Remove the cold water inlet union at the Viessmann hydraulic block. Take out and clean the mesh filter under running water to remove debris.
• Inspect flow sensor: Locate the DHW flow sensor (often integrated into the cold inlet assembly). Remove it carefully and check that the turbine or paddle moves freely. Descale with vinegar or a mild descaling solution if stiff.
• Check sensor wiring: Ensure the flow sensor connector is fully seated on the main PCB and that the cable is not pinched or damaged.
• Test operation: Reassemble, open the inlet valve, and purge air by running a hot tap. Restore power and check if the unit now detects flow and starts without F9.
• Replace sensor if needed: If the unit still does not register flow in diagnostics, replace the DHW flow sensor assembly with a compatible Viessmann part.