Geo
Manual Fan (G) Control Failure
Thermostat does not turn on the fan in 'Fan On' mode, but fan runs during heating or cooling.
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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 output not configured for manual fan, Firmware bug in fan control logic, Failed G relay on Geo PCB, Misinterpretation of fan mode behavior
How to Fix / Troubleshooting
Safety first: Turn off HVAC power at the breaker before moving wires if needed.
- Step 1 – Verify fan mode: On the thermostat, set fan mode to On or Circulate and wait a few minutes to see if the fan engages.
- Step 2 – Confirm G wiring: Ensure the G wire is connected to the G terminal at both the thermostat wall plate and the furnace control board.
- Step 3 – Test fan with jumper: With power on, briefly jumper R to G at the furnace control board. If the fan runs, the furnace and wiring are good.
- Step 4 – Firmware update: Update the thermostat firmware to correct any known fan control issues.
- Step 5 – Replace thermostat: If the fan never runs in manual mode despite correct wiring and firmware, the G relay on the Geo thermostat may be defective. Replace the thermostat head unit.
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Repair Difficulty
Medium
3/5
Required Part
G Fan Call Circuit / Main Control PCB
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