F05 error and the unit reports low salt even when the brine tank is full

Safety: Disconnect power. Wear gloves when working inside the brine tank.

Procedure:

• Verify actual salt level: Remove the brine tank lid and visually confirm that salt pellets are present and not bridged above the water level.
• Locate the salt sensor: Depending on the Vaillant model, this may be a float switch in the brine well or an electronic probe on the side of the tank. Gently lift the float to ensure it moves freely.
• Clean the sensor: Remove the float or probe and clean off any sludge, iron deposits, or scale using warm water and a soft brush. Avoid harsh abrasives.
• Inspect wiring: Trace the cable from the sensor to the control head. Check for cuts, crushed sections, or loose connectors. Reseat the plug on the PCB.
• Functional test: With power restored, manually actuate the float (or bridge the sensor contacts briefly if safe and per service documentation) to see if the display responds. If not, replace the salt level sensor assembly.
• Reset alarm: After cleaning or replacement, clear the F05 code and monitor the next few regeneration cycles to ensure the warning does not return.