TP-Link Kasa Fan Control (G Circuit) Misconfiguration or Wiring Fault

Thermostat fan setting does not work as expected (fan won’t turn on in ON mode or runs constantly in AUTO)

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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

Loose or missing G wire, Misconfigured fan control in thermostat, Stuck fan relay on furnace board, Incorrect system type selection

How to Fix / Troubleshooting

Safety: Turn off power to furnace/air handler before wiring checks.

  • Check G wiring: At thermostat and furnace board, ensure the G conductor is connected and secure.
  • Fan mode test: With power restored, set fan to ON in the thermostat. If the blower does not start, the issue is in wiring, thermostat G output, or furnace fan relay.
  • System settings: In the Kasa app, verify fan control options and ensure the thermostat is set to control the fan (if your system requires it).
  • Technician test: A technician can jumper R to G at the furnace board. If the blower runs with the jumper but not via thermostat, replace the thermostat. If it does not run, the furnace fan relay or control board is faulty.
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Repair Difficulty

Hard 4/5

Required Part

Fan Call (G) Circuit / Furnace Fan Relay
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