E7
Nest Common Wire Detection / Power Sharing Fault

Nest thermostat displays E7 and reports a C wire or power sharing issue

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

C wire connected at Nest but not at furnace control board, Loose C terminal screw at furnace, Shared C wire used for external accessory (humidifier/dehumidifier) causing voltage drop, Incorrect installation of Nest Power Connector

How to Fix / Troubleshooting

Safety: Turn off power to the HVAC system at the breaker before working on control wiring.

  • Trace the C wire: At the Nest base, note which color is on C. At the furnace control board, confirm the same color is attached to the C terminal. If it is not connected, attach it securely.
  • Tighten connections: Ensure the C terminal screw on the control board is tight and the conductor is not frayed or broken.
  • Check accessories: If a humidifier, dehumidifier, or other accessory shares the C terminal, verify its wiring is correct and not back‑feeding or overloading the transformer.
  • Verify Nest Power Connector: If installed, confirm the Power Connector leads are on the correct R, C, and call terminals per the Nest wiring diagram. Miswiring can cause E7.
  • Test voltage: With power restored, measure between R and C at the Nest base. You should see ~24 VAC. If voltage is unstable or drops when the Nest calls for heat/cool, the transformer or wiring may be undersized or faulty.
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Repair Difficulty

Medium 3/5

Required Part

C Wire (Common) or Nest Power Connector
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