Completed furnace, maintenance and inspection. I checked components and amp draws. I tested airflow and temp splits. I cleaned off flame sensor. I performed three safety tests on the furnace. Carbon monoxide is currently reading at 4 ppm. Furnace passed combustion analysis. During the visual safety check I found the seams on the heat exchanger very rusted. There are hooks coming off of the seam which means that the heat exchanger is starting to get eaten away at from the rust. I showed customer pictures of the heat exchanger and explained the caution of the current condition of the furnace. Although furnace passed the test today, due to current condition, I cannot guarantee safe operation through the winter. Furnace is operating at departure.