Nest thermostat displays E5 and may intermittently lose power or reboot

Safety first: Turn off power to the HVAC system at the breaker before modifying wiring.

• Check for C wire: Remove the Nest display and look for a wire connected to the C terminal. If there is an unused wire in the wall bundle, you may be able to repurpose it as a C wire.
• Verify C at control board: At the furnace/air handler, locate the C terminal on the low-voltage strip. Connect the same-color wire used at the Nest C terminal to the C screw and tighten securely.
• Confirm R and C continuity: Ensure the R and C wires are not loose, corroded, or pinched. Poor connections can cause voltage drop and Nest power issues.
• Reattach Nest and charge: After wiring C, restore power and reattach the Nest display. Allow up to an hour for the internal battery to charge. The E5 error should clear once stable power is detected.
• Alternative (Nest Power Connector): If you cannot run a C wire, consider installing a Nest Power Connector at the furnace/air handler following Google's wiring diagram. This provides a stable power path without adding a new cable.

If E5 persists even with a proper C connection, the HVAC transformer may be undersized or the Nest thermostat hardware may be defective.