I completed a diagnostic call on a approximately 40 to 50 year old furnace that is not working. The pilot is not lit. I took off the cover to access the burner and pilot area and found that this furnace is in very poor condition, heat exchanger is completely rusting out, evidence of flame rollout near the right side of the burner enclosure. I cleaned thermocouple and relit pilot and it stayed lit. Furnace is working on my departure but I told customer due to the age and the condition I cannot give him any guarantees he won’t have further issues. If the pilot continues going out it is probably being blown out. Provided customer with estimate on a replacement furnace.