Smart thermostat shows F.06 and cannot read the external outdoor sensor; weather compensation not working.

Safety: Isolate power to the boiler and controller before working on low‑voltage terminals.

• 1. Locate the outdoor sensor: Find the Vaillant external sensor (usually a small wall‑mounted box on a north or northwest wall). Inspect for physical damage or water ingress.
• 2. Check wiring at controller: At the smart thermostat or system controller (e.g. sensoCOMFORT), verify the two sensor wires are firmly connected to the designated outdoor sensor terminals.
• 3. Test resistance: With power off, disconnect the sensor wires and measure resistance across the sensor. Compare the reading to Vaillant’s NTC resistance chart for the current outdoor temperature. Infinite resistance indicates an open circuit; near 0 Ω indicates a short.
• 4. Inspect cable run: If readings are abnormal, check the cable for cuts, crushed sections, or water‑filled junction boxes. Repair or replace damaged cable.
• 5. Replace sensor: If the cable is sound but resistance is out of spec, replace the Vaillant outdoor sensor with the correct part number.
• 6. Clear error: Restore power and check the thermostat menu to confirm the outdoor temperature is now displayed correctly and F.06 has cleared.