A good place to look is to remove the blower assembly and look up into the bottom of the heat exchanger. If it has started leaking or coming apart, it will be pretty obvious. Many times it will give a pressure switch failure code as well. Don't have the codes here with me at home but should be on the inside of the blower panel the number of flashes. If the exchanger is bad, at that point, you would need to stop and find a dealer to take care of that unit for you. Or, start looking at a new furnace.