Nest thermostat shows E15 and reports that it has detected a serious power issue and has shut down to protect itself

Critical safety warning: E15 can indicate dangerous wiring. If you suspect any 120/240V wiring is involved, do not continue. Turn off power at the main breaker and contact a licensed HVAC/electrical professional.

• Power down: Turn off all HVAC breakers (furnace/air handler and outdoor unit). Remove the Nest display from the base.
• Inspect wiring type: Confirm that only small‑gauge (18–22 AWG) low‑voltage thermostat wires are connected to the Nest base. If you see thick Romex‑style cable or 120/240V conductors, the system is incompatible with Nest.
• Check transformer output: A technician should measure the transformer output; it should be around 24 VAC. Anything significantly higher can damage the Nest.
• Inspect control board: Look for burn marks or melted areas on the furnace control board, which indicate a short or surge.
• Replace damaged components: If the Nest was exposed to overvoltage, it may be permanently damaged and require replacement after the wiring and transformer are corrected.