Thermostat displays E9 and may reboot or freeze intermittently

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

Steps to troubleshoot:

• 1. Power cycle: Turn off the breaker for 5 minutes, then restore power. Observe if E9 reappears.
• 2. Factory reset: From the thermostat settings, perform a full factory reset. Reconnect to Wi‑Fi and reconfigure schedules and system type.
• 3. Check for overheating: Ensure the thermostat is not installed on an exterior wall with direct sun exposure or above heat sources (e.g., lamps, electronics). Excess heat can stress internal components.
• 4. Update firmware: Using the Emerson app, check for and apply any available firmware updates, following on-screen instructions carefully. Do not cut power during the update.
• 5. Persistent E9: If the error returns after reset and update, the internal memory or processor is likely failing and the thermostat should be replaced.

When to call a professional: If the thermostat controls complex multi-stage or heat pump systems, have an HVAC technician handle replacement and reconfiguration.