Furnace clean and tune. Cycled system. Checked heat exchanger. Found heat exchanger very rusted and corroded with signs of cracks starting . Checked burners and ignition system, both are worn. Checked natural draft box and found some traces of CO in box. This system does not have a motor for exhaust and system is open to exhaust. This runs a high risk of CO leaking into the house with higher winds or blockage. No CO currently found in airstream. This system is very worn. High risk of failure. This system also lacks safeguards to ensure safe operation. Highly recommend second opinion for options. Job complete. Office to bill.