Thanks. Those are good ideas.
Yes, a cracked heat exchanger is the end of the road for furnaces. I wonder if that is a replaceable part, too?
Of all things today, the exhaust inducer fan would not shut off after the burner cycle ended!
Heat exchangers wouldn’t typically crack unless they’re cast iron. More likely to see one burn through.
As for replacing one, it can be done, though you’d have to tear deep into the unit; like open heart surgery. Still, with photos, and an organized space to lay out all the parts as you remove them, reassembly should be just a matter of time and care. Biggest problem is getting it out where you can work on it, and putting it back when you’re done.
I think the control that runs the exhaust fan continues past the end of combustion either friends on a timer, a temperature sensor, or other detector. I’ve had one run on like that quite some time ago, and I recall a bad sensor of some kind had failed to tell the controller that it was OK to shut the fan off.
Good luck with your balky fan.