E9 error and thermostat shows abnormally high or low humidity readings, may disable humidity-based features

Safety: Turn off power to the thermostat at the breaker before removing it from the wall.

• Environmental check: Ensure the thermostat is not installed in a bathroom, above a humidifier outlet, or where steam regularly contacts the device.
• Gentle cleaning: Remove the thermostat and gently blow out dust from the vents using dry compressed air. Do not spray cleaners or water into the device.
• Power cycle and reset: Restore power and perform a factory reset via the Kasa app. Reconfigure and compare humidity readings to a known-good hygrometer.
• Observe over time: If humidity readings remain stuck or wildly inaccurate and E9 persists, the humidity sensor is defective.
• Resolution: The humidity sensor is integrated into the main PCB and is not field-replaceable. Replace the thermostat.