Thermostat displays E4 and may show incorrect humidity readings or fail to control dehumidification/humidification.

Safety first: Turn off power to the HVAC system at the breaker before removing the thermostat from the wall.

• Step 1 – Power cycle and reset: Remove the thermostat from the wall plate for 1 minute, then reinstall. If available, perform a factory reset from the settings menu (note: you may need to reconfigure Wi‑Fi and system settings).
• Step 2 – Inspect for moisture: Check the wall behind the thermostat for signs of condensation or water intrusion. If the wall is damp, address the moisture source and allow the area to dry completely.
• Step 3 – Clean vents: The humidity sensor often sits behind small vents on the thermostat housing. Use a soft brush or compressed air (short bursts) to remove dust buildup that can affect readings.
• Step 4 – Verify readings: Place a standalone hygrometer near the thermostat and compare humidity readings. If the thermostat is significantly inaccurate and E4 persists, the internal sensor is likely defective.
• Step 5 – Replace thermostat: The humidity sensor is typically integrated; if failed, replace the Carrier smart thermostat.
• When to call a pro: If humidity control accessories (like a Carrier humidifier or dehumidifier) are also malfunctioning, have a technician verify wiring and accessory modules before replacing the thermostat.