Furnace clean and tune. Cycled system. Checked heat exchanger, found large amount of rust. Checked and cleaned burners and flame sensor. Checked igniter, found it worn and cracks forming. Tested ok at this time. Found ventor motor bearings very worn. Tested airstream and found no CO at this time. Slightly elevated levels of XS air in exhaust. Normal CO levels in exhaust. Average life of this unit is 14 years. This system is showing signs of being worn. Future failure is expected. Job complete.