Display shows F12 and unit reports low salt even when tank is full

Safety first: Disconnect power before opening the brine tank and accessing any wiring.

• Clean around sensor: Locate the salt level sensor (float or electronic probe) in the brine tank. Remove any heavy salt crust or bridging around it.
• Check sensor movement: If it is a float type, move it up and down by hand to ensure it is not stuck. For electronic probes, clean the surfaces with warm water.
• Inspect wiring: Follow the cable from the sensor to the control board. Ensure connectors are fully seated and wires are intact.
• Reset alarm: Restore power and clear the F12 alarm via the control panel. Observe whether the correct salt status is displayed.
• Replace sensor if needed: If F12 returns despite cleaning and wiring checks, replace the Viessmann salt level sensor. Persistent issues may indicate a control board fault.