Vaillant smart thermostat shows F.05 and the room temperature reading is clearly wrong (e.g. 5°C or 50°C indoors).

Safety: Turn off power to the thermostat circuit if it is hard‑wired. For battery models, remove batteries before opening the housing.

• 1. Check installation position: Ensure the thermostat is not installed above a radiator, near a heat source, in direct sunlight, or in a draught. Relocating the thermostat can resolve apparent sensor faults.
• 2. Inspect for moisture or damage: Remove the thermostat from its base. Look for condensation, water marks, or corrosion on the PCB. If moisture is present, allow it to dry completely and correct the source of damp.
• 3. Clean contacts: For clip‑on wall plates, gently clean the contact pads with a dry cloth to ensure good electrical contact.
• 4. Compare with another thermometer: Place a reliable thermometer next to the thermostat and compare readings. If the thermostat is off by more than a few degrees and F.05 is shown, the internal sensor is likely defective.
• 5. Sensor calibration (if available): Some Vaillant models allow a temperature offset in the settings. Apply a small correction if the error is minor and stable.
• 6. Replace thermostat: If the sensor is clearly faulty or F.05 persists, the internal NTC cannot usually be replaced separately; replace the complete Vaillant room thermostat.