Nest thermostat shows E2 and will not operate the HVAC system

Safety warning: Turn off power to both the indoor air handler/furnace and the outdoor AC condenser at the breaker panel before working on wiring.

• Inspect Y1 at Nest base: Remove the Nest display and gently tug the Y1 wire. If it moves, re-strip 3–4 mm of insulation and reinsert fully while pressing the Y1 connector button.
• Trace Y1 at HVAC equipment: At the furnace/air handler, locate the thermostat terminal strip and verify the Y (or Y1) wire from the thermostat cable is firmly attached. Tighten the screw if loose.
• Check outdoor unit connections: At the condenser (power off), open the control panel and confirm the low-voltage Y and C wires from the furnace are secure and not corroded or broken.
• Inspect for shorts: Look for any spots where the thermostat cable's Y1 conductor may be pinched, cut, or touching bare metal. Repair with wire nuts or replace the cable if damaged.
• Restore power and test cooling: Reattach the Nest display, turn breakers back on, and set the thermostat to cool. If Y1 power is restored, the E2 code should clear.

If the error remains, the outdoor contactor coil or control board may be open and should be tested by an HVAC technician.