Blower on furnace works in on position only not in auto position?

Answer 1

Check the Thermostat or Thermocouple(little probe-like with usually a copper wire coming from it going to the switch or thermostat). It is probably touching somthing hot and telling the blower that it is warm enough and don't come on or either it just isn't sending a signal to turn on at all.

I know one of ours got covered up with dust and was getting warm and because of the dust was acting like an insulation, the thermocouple was staying warm longer than usual, it wasnt allowing the heater to kick on again.

And in one heater of ours the thermocouple was bent right in the line of where the hot air blows out instaed of the return ar and it would come on and almost kick right back off. The thermocouple should have been where the air return air was so it could tell when the air inside the room was warm then kick the heater off.