Thermostat displays E3 and may show 'Humidity sensor error' or inaccurate indoor humidity readings.

Safety first: Switch off HVAC power at the breaker before removing the thermostat.

Steps:

• Remove thermostat from wall: Gently pull the Rheem smart thermostat off its sub-base to expose the back and internal components.
• Inspect for moisture: Look for signs of condensation, water stains, or corrosion around the humidity sensor area (often a small vented module).
• Clean sensor vents: Use a can of dry compressed air to blow dust out of the sensor vents. Do not use liquid cleaners.
• Check connectors: If the humidity sensor is on a small daughterboard or ribbon cable, ensure the connector is fully seated.
• Reinstall and power up: Reattach the thermostat, restore power, and wait 10–15 minutes for humidity readings to stabilize.
• If E3 persists or humidity reads 0% or 99% constantly: The humidity sensor module is likely failed. Replacement of the thermostat or the internal humidity sensor board is required and is best handled by a technician.