Thermostat shows E4 and system may shut down or operate erratically based on humidity

Safety: Turn OFF power to the HVAC system at the breaker before removing the thermostat.

• Visual inspection: Remove the thermostat from the wall plate and inspect the vented area where the humidity sensor resides. Look for dust buildup, corrosion, or signs of moisture.
• Gentle cleaning: Use dry compressed air to blow out dust from the sensor vents. Do not insert tools or use liquids, as this can further damage the sensor.
• Disable humidity control (temporary): If the thermostat allows, disable advanced humidity control features in the settings so the system can operate based primarily on temperature while you arrange repair.
• Reboot: Power cycle the HVAC system and thermostat by turning the breaker OFF for 2 minutes, then ON. Check if the E4 code clears.
• Replacement: If the humidity reading is stuck (e.g., 0% or 99%) or wildly inaccurate and E4 persists, the internal humidity sensor is defective. Replace the thermostat with a new Ruud EcoNet smart thermostat.