F15
Vaillant Low Voltage Sensor Supply Fault

F15 error and the softener reports sensor supply fault or multiple sensor errors simultaneously

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Failed low-voltage regulator on PCB, Short circuit in sensor wiring, Water ingress causing tracking on PCB, Incorrect external wiring to sensor terminals

How to Fix / Troubleshooting

Safety: Isolate mains power. Internal PCB work should only be done by competent persons.

Steps:

  • Check for moisture: Inspect the control housing and PCB for signs of condensation or leaks. Dry thoroughly and correct any plumbing leaks above the unit.
  • Inspect sensor wiring: Disconnect non‑essential sensors (salt level, external inputs) one at a time and see if the error clears when power is restored. A shorted sensor can pull down the supply.
  • Look for damage: Examine the PCB for burnt components near the low‑voltage regulator section. If present, the board must be replaced.
  • Do not bypass fuses: If the board has internal fuses, never bypass them; replace only with identical types.

Recommendation: F15 usually requires PCB diagnosis and replacement by a Vaillant‑authorised service engineer.

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Repair Difficulty

Professional Required 5/5

Required Part

Control PCB Low-Voltage Supply Section
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