I have a gas furnace that will only work when the fan is on "ON'. I changed the thermostat, same problem. Changed the circuit board, same problem. Changed the ignition modulator, same problem. Changed the fan motor, same problem. Any ideas?

Sounds like it would have been cheaper to have changed the entire furnace.

Can you tell me the brand of your furnace and about what is the age of the furnace.

Also tell me what happens if the thermostat calls for heat and the fan is on auto.

The furnace is a Heil I believe the furnace to be between 6 -7 yrs old.
When the thermostat calls for heat and the fan is on auto you can hear the furnace ignite but the fan will not start. It may be the fan I am hearing trying to kick in I know nothing about a furnace.You can however turn the fan to on and wait a few minutes then turn back to auto and it will cycle 1 time but will not come on again when the thermostat is calling for heat.

The way you described it I think that you have a relay out on the circuit board. Some boards have plug-in relays, I am not sure about yours. You might have to change the complete board.

Even if we just replaced the circuit board? We changed the flame sensor today.