How to Fix / Troubleshooting

Safety first: Switch off power to the furnace/air handler at the breaker before opening any panels.

Check G wire at Nest: Remove the Nest display and verify the G wire is fully seated in the G terminal. Re-strip and reinsert if the copper is short or corroded.
Inspect furnace control board: Remove the blower compartment cover. Locate the G terminal on the low-voltage strip and confirm the thermostat G wire is firmly attached and not frayed.
Test fan operation: With power restored, temporarily jumper R to G at the furnace terminal strip (only if you are comfortable and understand low-voltage circuits). If the blower does not start, the fan relay or blower motor circuit may be faulty.
Remove jumper and restore normal wiring: Turn power off again, remove any test jumper, and ensure all wires are back in their correct terminals.

If the fan relay on the control board is defective, the board will likely need replacement by a qualified technician.

Nest thermostat displays E3 and cannot control the fan

Possible Causes