Smart thermostat shows F.14 and refuses to start heating because of a detected wiring fault.

Safety: This error often indicates dangerous wiring. Isolate the boiler and heating system at the main switch and verify power is off before inspecting.

• 1. Compare with wiring diagram: Obtain the Vaillant boiler and thermostat wiring diagrams. Verify each conductor is on the correct terminal (eBUS vs 230 V vs room stat contacts).
• 2. Check for 230 V on low‑voltage lines: Use a multimeter to ensure there is no mains voltage present on eBUS or sensor terminals. If mains is detected, disconnect and correct the wiring immediately.
• 3. Inspect wiring center: Open the wiring center and check for crossed wires, loose strands touching adjacent terminals, or damaged insulation.
• 4. Remove third‑party controls: Temporarily disconnect any non‑Vaillant controls to simplify the system and retest.
• 5. Replace damaged parts: If a low‑voltage board has been exposed to mains, the thermostat or boiler PCB may be irreparably damaged and must be replaced by a qualified engineer.