Welcome to Just Answer. I will do my best to fully answer your question(s).
For this brand make and model its had a good life apparently, Sorry to say! You have done all the normal things one can do to trouble shoot these (and most) units for the most part. Its likely that the internal components are either worn out or gummed up. Or the circuit board like you mentioned. Not can you get replacement parts? Maybe. But then what happens the next week to it. I recommend shopping for a new unit. I know a contractor supplied one will cost more but at least THEY carry a warranty for the unit and later on when that expeires the contractor is familiar with the unit and likely will have repalcement parts for you. Might get 15 -20 yrs out of that. Just food for thought.
You are welcome. I am not a big sears fan of any product sorry to say! Please remember to hit the ACCEPT button.