When my furnace starts up and heat up, the fan does not start automatically. If I ran the fan, it will start. What should I do. Last time it did that, I had to only change batteries. HELP its getting cold.

What do you mean by ran the fan(turn thermostat to on instead of auto?) You may have a bad control board or a bad heat lead wire to your motor. Please give make model and year of furnace.

Possibly a bad limit control. In certain models the plenum temperature turned on and off the fan. In later furnaces they just use time.