Display shows F4 and softener is stuck in one regeneration step

Safety first: Disconnect power and isolate the softener from water pressure before opening the valve head.

• Locate position sensor: On Viessmann softeners, a microswitch or encoder is mounted near the valve cam. Remove the valve head cover to access it.
• Check alignment: Verify that the cam or magnet that actuates the sensor is correctly aligned and not cracked or loose on the shaft.
• Inspect wiring: Follow the harness from the sensor to the control board. Look for broken insulation, loose connectors, or corrosion at the plug.
• Test sensor operation: With power restored and the cover removed, carefully initiate a manual regeneration and observe whether the sensor is being actuated as the cam rotates. Use a multimeter to check continuity on a microswitch or signal output on an encoder if you are qualified.
• Replace faulty parts: If the sensor does not change state as the cam moves, replace the Viessmann valve position sensor / encoder. If the sensor works but F4 persists, the control board may require replacement.