Nest thermostat displays E1 and cannot control heating or cooling

Safety first: Turn off power to the furnace/air handler and AC condenser at the breaker panel before touching any thermostat or HVAC wiring.

• Check thermostat wiring: Remove the Nest display from the wall. Inspect the R (Rc or Rh) wire in the base. Ensure the bare copper is straight, 3–4 mm exposed, and fully inserted under the R terminal with the connector button fully depressed.
• Verify correct terminal: If you have a single R wire, it should go to Rc on most Nest bases. Confirm that it is not loose or in the wrong terminal.
• Inspect HVAC control board: At the furnace/air handler, remove the access panel (power must be off). Locate the low-voltage terminal strip (R, C, W, Y, G). Confirm the R wire going to the thermostat is firmly attached to the R screw and not broken.
• Check low-voltage fuse: Look for a 3A or 5A blade fuse on the control board. If blown, replace with the same rating only after correcting any shorted wires (inspect thermostat cable for damage or bare copper touching metal).
• Restore power and test: Turn breakers back on, reattach the Nest display, and wait a few minutes. The E1 code should clear if R power is restored.

If the error persists after confirming wiring and fuse, the HVAC transformer or Nest base may be faulty and a professional technician should test voltages.