F18 error and thermostat cannot read humidity or shows 0%/100% constantly

Safety first: Power off the thermostat if opening the casing.

Steps:

• 1. Compare readings: Use a separate hygrometer near the thermostat. If the thermostat shows 0% or 100% and F18, the sensor is likely faulty.
• 2. Inspect for moisture damage: Remove the thermostat and look for signs of condensation or corrosion around the small humidity sensor chip (often near vent holes).
• 3. Dry out: If recently exposed to high humidity, allow the thermostat to dry in a warm, dry room for several hours, then power back on.
• 4. Firmware update: Check for firmware updates via the Ideal app that may address sensor issues.
• 5. Replace thermostat: The humidity sensor is usually integrated and not replaceable separately.