Rising utility bills. You may notice the furnace blower operating excessively or cycling on and off repeatedly. Your utility expenses may climb as a result of your furnace running longer to deliver the same amount of heat. A yellow or flickering flame might indicate that the furnace is emitting toxic carbon monoxide, a very serious sign that you must replace your furnace. Odd noises such as pounding, cracking, rattling or screeching. These might indicate that your furnace needs to be replaced.