Amazon Smart Thermostat
Stuck Fan Relay or G Circuit Short
Fan runs continuously even when thermostat is set to Auto and no heating or cooling is requested
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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
G wire shorted to R at furnace or in cable, Furnace control board fan relay stuck closed, Thermostat fan output stuck ON
How to Fix / Troubleshooting
Safety: Turn off furnace/air handler power at the breaker.
- Check thermostat settings: Confirm the fan mode is set to Auto, not On.
- Inspect G wiring: At both thermostat and furnace, ensure the G wire is not touching R or other terminals.
- Test board vs thermostat: With power off, disconnect the G wire from the furnace control board. Restore power. If the fan still runs, the furnace control board relay is stuck. If the fan stops, the issue is in the thermostat or wiring.
Replace the furnace control board if its fan relay is stuck, or replace the thermostat if its G output is stuck ON and wiring is good.
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Repair Difficulty
Hard
4/5
Required Part
Furnace Control Board or Amazon Smart Thermostat Main Control Assembly
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