Thermostat displays E3 and cannot detect HVAC system correctly

Safety: Turn off power to the furnace/air handler and outdoor unit at the breaker before touching any thermostat wiring.

• Verify system compatibility: Confirm your HVAC system is a standard 24V system (conventional or heat pump). The Amazon Smart Thermostat is not compatible with proprietary communicating systems.
• Check wiring against the old thermostat: Take a photo of the old thermostat wiring (if available). Match each wire to the correct terminal on the Amazon Smart Thermostat wall plate (R/Rc, W, Y, G, C, O/B, Aux, etc.).
• Reseat wires: For each connected wire, press the terminal release, pull the wire out, trim and re-strip to about 1/4 inch (6 mm), then reinsert fully and tighten/lock the terminal. Tug gently to ensure it is secure.
• Remove unused or miswired conductors: Ensure no bare copper is touching adjacent terminals. Cap or tape unused wires.
• Re-run setup: Restore power and run the guided setup in the Alexa app so the thermostat can re-detect the system.

If E3 remains after correcting wiring and confirming compatibility, the wall plate terminal block or internal detection circuit may be faulty and the thermostat should be replaced.