Thermostat displays E1 and will not control heating or cooling

Safety first: Turn off power to the HVAC system at the breaker before removing the thermostat from the wall to avoid shorting low-voltage circuits.

Steps to troubleshoot:

• 1. Power cycle the thermostat: Turn HVAC breaker off for 2 minutes, then back on. If E1 clears temporarily and returns, the sensor circuit is likely faulty.
• 2. Inspect thermostat mounting and wiring: Remove the Emerson thermostat from its wall plate. Check that the thermostat is fully seated on the sub-base and that the low-voltage pins are not bent or corroded.
• 3. Check for physical damage: Look for signs of liquid intrusion, impact damage, or burnt components near the internal temperature sensor area (usually near the bottom or side vents).
• 4. Reset to factory defaults: From the settings menu, perform a factory reset (refer to the specific Emerson model manual, e.g., Sensi ST55, Sensi Touch ST75). After reset, reconfigure and see if E1 reappears.
• 5. If E1 persists: The internal sensor or its circuit on the main PCB is likely open. This is not field-repairable on most Emerson smart thermostats; replacement of the thermostat is typically required.

When to call a professional: If you see burn marks, smell electronics burning, or the error returns immediately after reset and reseating, contact an HVAC technician or Emerson support for thermostat replacement.