Johnson Controls
Fan Control Circuit Failure
Thermostat indicates fan is set to ON, but the indoor blower does not run.
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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
Open G conductor, Miswired fan terminal at air handler, Failed fan relay on air handler control board, Thermostat not actually energizing G output
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler before working on wiring.
- Step 1 – Check thermostat settings: Confirm the fan mode is set to ON or CIRC and not AUTO.
- Step 2 – Inspect wiring: Verify the G wire is connected at both thermostat and air handler control board.
- Step 3 – Voltage test: With fan ON, measure between R and G at the air handler. If 24 VAC is present but the fan does not run, the issue is in the air handler fan relay or motor.
- Step 4 – Technician service: Have a technician test and replace the fan relay or blower motor as needed.
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Repair Difficulty
Hard
4/5
Required Part
Air Handler Fan Relay / Blower Motor (not thermostat)
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