I have an older A/C and furnace unit in my attic; the A/C is electric and the furnace is gas. I had the blower fan changed on the unit in August. When the weather became cold, I couldn't get the heater to start. I thought the pilot light may have been out since the blower fan was replaced. It turned out that the pilot was lit, but the breaker had been tripped. After resetting the breaker, I was still unable to get the heater/furnace to work, unless I put the fan in the on mode. The auto mode will not work for the furnace, but it will work for the A/C. Any suggestions?

Does the funace actually fire but the fan not come on? To find out, set the thermostat well above room temperature then go into the attic to see if the burners came on.

Yes, furnace does fire in either mode, but fan does not come on when in auto mode for furnace, only A/C auto.

I suspect you have a bad fan relay. It could be on a furnace control board or if the furnace is really old, a separate fan control.