You didn't mention cleaning the contacts on both sides of the TC, but I assume you did that. Make sure the camera is fully seated on the TC with the lock pin clicked in, and that the TC is fully seated to the camera with the lock pin clicked in. If either is a little off, it can cause an f/00 even though AF will work. If nothing works, sending the TC to Canon sounds like the only remaining option left (unless someone else has another idea).