Bosch
Fan Only Mode Not Working
Thermostat does not turn on the fan in 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 (fan) wire not connected at thermostat or control board, Fan mode disabled in installer settings, Faulty fan relay output on thermostat, Control board not configured to accept G signal
How to Fix / Troubleshooting
Safety: Turn off power to the air handler/furnace before working on wiring.
- Verify G wiring: Confirm the G wire is connected at both the thermostat and the air handler/furnace control board.
- Installer settings: In the thermostat’s advanced menu, ensure fan control is set to "thermostat" if the system expects the thermostat to control the fan.
- Bypass test: With power on, jumper R to G at the control board. If the fan runs, the board and blower are functional.
- Test thermostat output: Set fan to ON and measure for 24 V between G and C at the control board. If no voltage, the thermostat output may be faulty.
- Replace thermostat if needed: If wiring and board are good but G is never energized, replace the thermostat.
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Repair Difficulty
Medium
3/5
Required Part
Bosch Smart Thermostat Main Control Assembly
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