Nest thermostat shows E6 and cannot communicate with the HVAC system correctly

Warning: Never connect a Nest thermostat directly to high-voltage (120/240V) heater circuits. It is designed for 24V systems only.

• Review system type: Confirm your HVAC system is a standard 24V forced-air, heat pump, or boiler system. If it is line-voltage baseboard, a proprietary communicating system, or uses more than 30 VAC, Nest is not compatible.
• Check wiring labels: Remove the Nest display and compare each wire's terminal (R, C, Y1, W1, G, O/B, etc.) to the labels on the furnace/air handler control board. Each conductor should match the same function at both ends.
• Correct miswired heat pump circuits: For heat pumps, ensure the reversing valve wire is on O/B, and that Y1 is the compressor call. Do not combine O/B with Y1 on a single conductor.
• Run additional conductors if needed: If multiple functions were combined on one wire with a previous thermostat, you may need to pull a new thermostat cable with more conductors or use a Nest Power Connector or other approved interface.
• Redo Nest setup: After correcting wiring, go to Settings > Equipment on the Nest and re-run the equipment detection and configuration wizard.

If your system is proprietary (e.g., uses a data bus instead of R/C/Y/W), Nest will not work and you should revert to the original thermostat or use a compatible adapter recommended by the HVAC manufacturer.