I have an intertherm central air/heat unit. The ac works fine. All of a sudden today the heater will come on but the fan won't blow. If I turn it on run the fan does, but on auto it doesn't. It still works fine when the ac unit is on.

The fan control is defective. Post the exact model number of your furnace.

Turned out to be a sequencer was going bad on the heater unit. Replaced it and now it is running fine. Thanks for your advice.

Cool the heat sequencer is actually the fan control on most models of intertherm