Thermostat displays E9 and may reboot repeatedly

Safety first: Turn off power to the HVAC system at the breaker before extended troubleshooting to avoid repeated power cycling of the furnace board.

Steps to attempt:

• Stable power: Ensure the HVAC breaker is firmly on and not tripping. Avoid rapidly switching the breaker on and off.
• Hard reset: Remove the thermostat from the wall plate for 5 minutes. Restore it to the wall, then restore HVAC power. Allow the thermostat to fully boot without pressing buttons.
• Check for firmware update: If the thermostat can connect to Wi‑Fi and the app, check for and apply any available firmware updates through the Sensi app.
• Factory reset: Perform a factory reset from the thermostat menu or app. Reconfigure the system and observe if E9 returns.

When to replace: If E9 persists after reset and firmware update attempts, the internal memory or processor is likely defective. The thermostat assembly should be replaced.