Welcome to the forum.
Do you want to replace the exhaust fan or the microwave?
I saw in another one of your posts you have a'24 Solitude. It should still be under warranty. Maybe talk to GD's customer support folks.
Kind of odd that it works when boiling water on the stove and the fan above in the microwave then works, but it doesn't work by itself. There really isn't any apparent relationship between the two appliances.