I bet first that they are referring to the I/O board which is featured [guide|16946|in this repair guide].
Second, I bet that they found liquid damage on that board which is why they refused to warranty it. Or not? If not, what caused your warranty to be void?
If it was liquid damage indeed, damage could be on the main board, not just I/O board. Or even both.
So history is important and you'll need to provide it if you need more help from us to determine what may be wrong.