Nest thermostat shows E2 and reports a sensor error or temperature reading failure

Safety: Turn off power to the HVAC system at the breaker or service switch before removing the thermostat from its base.

• Restart and update: Restart the Nest (Settings > Reset > Restart). Connect to Wi‑Fi and check for firmware updates in the Nest app. Sometimes sensor calibration issues are corrected by updates.
• Check environment: Ensure the thermostat is not installed above a humidifier, kettle, or in direct sunlight, which can cause extreme readings. If there is visible condensation on or behind the thermostat, allow it to dry completely (power off, remove from base, dry in a warm, dry room for several hours).
• Inspect for damage: Remove the Nest from the wall plate and inspect the back and front edges for signs of corrosion, liquid stains, or warping. If present, the sensor circuitry is likely damaged.
• Compare readings: Use a separate room thermometer/hygrometer to compare readings. If the Nest is far off and E2 persists after restart and update, the internal sensor assembly has likely failed and the thermostat should be replaced.