Furnace clean and tune. Cycled system. Checked heat exchanger. Found deep etching and rust. Some evidence of possible cracks forming. Checked and cleaned flame sensor and burners. Igniter tested low. Preformed combustion test and found evidence of secondary heat exchanger issues. Found beatings are worn in ventor motor. No CO currently in airstream. Checked and tightened electrical connections and clamps. Average life expectancy for this unit is 14 years. This unit is 31 years old and in poor condition. Failure is expected. Job complete.