I just turned on my furnace for fall season - the fan/blower does not come on in the auto position but does work properly in the on position - question is , is this a sensor, motor or thermostat issue? Thank You

Depends upon the unit but there is a item called a fan control or relay that controls the blower in the heat mode. When you switch the thermostat to the ON position instead of auto you are using the A/C relay instead of the fan heat control.

On older units they look like this.