Nest thermostat shows E15 and may shut down to prevent damage

Danger: Overvoltage can permanently damage the Nest and pose a shock or fire hazard. If you suspect line voltage, do not touch bare wires.

• Turn off all related breakers: Immediately shut off power to the furnace/air handler, boiler, and any external transformers feeding the thermostat.
• Inspect wiring type: If the thermostat cable is connected directly to 120/240V heater terminals or uses thick gauge wire typical of power circuits, Nest is incorrectly installed and must be removed.
• Check transformer output (professional): A technician should measure the transformer secondary voltage; it must be around 24 VAC. Anything significantly higher indicates a faulty transformer.
• Replace damaged Nest: If E15 occurred due to overvoltage, the Nest hardware is often irreparably damaged and should be replaced after correcting the wiring.

Do not reconnect a new Nest until you are certain the thermostat circuit is a standard 24V low-voltage system.