No heat for the upstairs system, due to a dirty flame sensor. I was able to clean the flame sensor, and heat works. Inspected entire system. Found heat exchanger very rusted with crack, return plenum is extremely dirty and blower wheel is dirty, as well. Air quality in the house is compromised. Sent estimates for potential repairs (not recommend). At this age (furnace 25 years old and ac 6 years old), the best option would be to upgrade the system, for safety, comfort and efficiency purposes. No diagnostic fee. We did home care inspection this month. Set a lead to quote upgrading for the upstairs furnace for tomorrow after 12. We should credit 89,00 back, if customer replace system,