Furnace clean and tune. Checked heat exchanger, found several heavy rust spots and deep etching where cracks are forming. Found burners very rusted and corroded. Flame sensor/igniter is very worn. Ventor motor bearings are worn. Checked for CO in airstream, none found at this time. High XS air found in exhaust indicating that there is some blockage in secondary heat exchanger. CO levels in exhaust are also elevated. Average life of this unit is 14 years, this system is 17+ years old and in very poor shape. Failure is expected. Second opinion recommended. System operating normal at this time, however several issues found. Job complete.