My heating unit works. You can hear and smell the gas fire. However, when switched to Auto on the Thermostat the fan will not kick-in. I can switch the fan to Fan not Auto and the fan runs.

When I have the Heat and Auto switched on the Fan does not run. When I set the Thermostat to AC and Auto the Fan does work.

I have a Heating / AC tower in the Attic.

Is the Thermostat the problem or do I need service on the heating unit fan portion?

do I need service on the heating unit fan portion??

Yes the fan limit control is probably defective.