Thermostat displays E3 and system will not run correctly in heat or cool

Safety first: Turn off power to the HVAC system at the breaker before changing any wiring. Configuration changes in software can be done with power on, but wiring must be de‑energized.

Steps to resolve:

• Confirm HVAC system type: Identify whether your system is conventional (gas/oil furnace with AC) or a heat pump, and how many stages of heat/cool it has. This information is usually on the air handler or furnace nameplate.
• Run Sensi installer setup: Using the Sensi app or thermostat menu, enter the installer configuration. Verify system type, number of stages, fan control (gas vs electric), and reversing valve (O/B) settings match your equipment.
• Check O/B wiring for heat pumps: Ensure the reversing valve wire is connected to the correct O/B terminal on the Sensi wall plate and that the configuration matches whether your heat pump energizes the valve in heating or cooling mode.
• Verify compatibility: Compare your system type (e.g., multi‑stage heat pump with dual fuel) with the compatibility list for your specific Sensi model. Some advanced systems require a different thermostat or an interface module.
• Power cycle: After correcting configuration, turn the HVAC breaker OFF for 1–2 minutes, then ON. Allow the thermostat to reboot and re-test heating and cooling.

When to replace or call a pro: If E3 persists after correct configuration and your system is confirmed compatible, there may be an internal logic or relay fault on the thermostat main board. Replacement of the thermostat or evaluation by an HVAC technician is recommended.