Lennox Fan-Only Control Failure

Thermostat does not energize the fan in 'Fan On' mode, but fan runs during heating or cooling.

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.

Possible Causes

Incorrect fan mode configuration in Lennox thermostat, Failed G output in fan-only mode, Miswired fan relay on Lennox control board, Software bug affecting manual fan command

How to Fix / Troubleshooting

Safety warning: Turn off power to the indoor unit before working on wiring.

1. Verify fan mode:
- Ensure 'Fan On' or 'Circulate' is selected on the thermostat.
2. Check G wiring:
- Confirm G wire is connected at thermostat and Lennox control board.
3. Test fan relay:
- Since fan runs in heat/cool, the relay is likely good; issue may be thermostat output.
4. Replace thermostat if it fails to energize G in fan-only mode:
- Install a new Lennox thermostat if configuration is correct and wiring is intact.

Repair Difficulty

Medium 3/5

Required Part

Lennox Smart Thermostat

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