Thermostat displays E4 and may shut down heating or cooling

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

Steps to troubleshoot:

• 1. Verify model capability: Confirm your Emerson smart thermostat model (e.g., Sensi Touch ST75H) actually has built-in humidity sensing. If not, E4 may indicate a different internal fault—consult the model-specific manual.
• 2. Power cycle and reset: Turn off power for 2 minutes, restore power, then perform a factory reset from the thermostat menu. Reconfigure and check if E4 returns.
• 3. Inspect for contamination: Remove the thermostat from the wall plate and visually inspect the vented area where the humidity sensor resides. Look for dust buildup, paint overspray, or signs of moisture.
• 4. Clean gently: Use dry compressed air to blow out dust. Do not use water, solvents, or brushes on the sensor area.
• 5. Disable humidity control (temporary workaround): In settings, turn off any dehumidification or humidification control features to allow basic temperature control while you arrange replacement.
• 6. Persistent E4: Internal humidity sensors are not field-replaceable; the thermostat typically must be replaced.

When to call a professional: If humidity control is tied to whole‑home dehumidifiers or humidifiers, have an HVAC technician verify those devices are wired correctly and not backfeeding voltage into the thermostat.