Honeywell Home
Fan Control Configuration or G‑Wire Issue
Thermostat does not turn on fan in Auto mode even though heating or cooling is running
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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
Fan set to be controlled by equipment, Loose G wire, Incorrect system type configuration, Failed fan relay on furnace board
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler before working on wiring.
- Test fan in On mode: Set the thermostat fan to On. If the fan does not run, the issue is likely wiring or the furnace fan relay.
- Inspect G terminal wiring: At the thermostat and furnace board, ensure the G wire is secure and not broken.
- Check system configuration: In installer settings, verify whether the fan is controlled by the thermostat or the equipment. For most forced‑air systems, the furnace controls the fan in heat mode.
- Test fan relay (advanced): With power off, jumper R to G at the furnace board. Restore power; if the fan runs, the relay is good and the thermostat or wiring is suspect. If not, the fan relay or blower motor circuit needs service.
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Repair Difficulty
Hard
4/5
Required Part
Furnace Fan Relay
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