Display shows F01 and the softener does not start a regeneration cycle

Safety first: Turn off electrical power to the softener and close the main water supply before dismantling any plumbing connections.

Step-by-step checks:

• Verify inlet valve position: Ensure the Vaillant softener's cold water inlet isolation valve is fully open. Many installations have a small quarter‑turn ball valve on the inlet line—handle should be in line with the pipe.
• Check household water pressure: Open a nearby cold tap. If flow is weak, the issue may be with the building supply, not the softener. Restore normal pressure before further diagnosis.
• Clean inlet filter screen: Shut off water to the softener, relieve pressure by opening a downstream tap, then disconnect the inlet union. Remove and rinse the small mesh filter screen in clean water to remove grit or limescale. Refit carefully, ensuring O‑rings are seated.
• Inspect inlet flow sensor: Many Vaillant softeners use a turbine or Hall‑effect flow sensor on the inlet block. Check for debris or scale around the turbine. If the turbine is jammed or the sensor wiring is damaged, replace the flow sensor module.
• Reset the control: Restore water and power, then clear the F01 code according to the user manual (usually by pressing and holding the reset or program key). Run a manual regeneration and confirm that the code does not reappear.

When to stop: If F01 returns immediately after cleaning the filter and confirming pressure, the inlet flow sensor or control board may be defective. In that case, contact a Vaillant‑authorised technician.