Diagnosis
The G terminal signal (Fan control) is not being sent or received. This is a wiring, configuration, or equipment issue.
How to Fix It
First, verify the wiring. Remove the thermostat from its base. Confirm a wire is securely inserted in the G terminal on both the thermostat and the baseplate. Next, test the fan manually. On the thermostat, press the FAN button until the display reads 'ON'. You should hear a click from the thermostat. Then, go to your furnace and listen for a second click from the control board relay. If you hear both clicks but the blower doesn't start, the issue is at the furnace.
If That Didn't Work
The problem is likely a faulty blower motor, control board relay for the fan, or a disconnected wire at the furnace. At the furnace control board, locate the G terminal. With the thermostat calling for FAN ON, use a multimeter to check for 24VAC between G and C. If voltage is present, the blower motor or its capacitor has failed. If no voltage, the thermostat wire between the G terminals is broken.
Heads Up
In 'Auto' mode, the fan only runs during a heating or cooling cycle. There is a built-in delay on most systems. Wait up to 90 seconds after the furnace or AC starts for the blower to engage. This is normal operation.