Completed furnace, maintenance and inspection. I cleaned flame sensor. I checked amp draws and components. I tested operation of the system. High inspected heat exchanger. Heat exchanger looks good from the outside with just a little bit of strain. The inside of the heat exchanger has Some deep rust developing. I showed customer pictures but with the other tests that we ran it showed that the heat exchanger was not open yet. Customer has carbon dioxide detectors on each floor of the house. Furnace is operating within manufacture specifications at departure.