Sounds like a duff box if it's giving the same error on your brothers (confirmed) working dish and LNB.
How long have you been with Sky? If it's less then a couple of years I'd get them to replace/repair the box for you as it's not fit for the purpose it was intended. If more then that then I'd buy a new box.
IF Sky give you hassle about replacing it within the first couple of years then have a read here:
http://cableforum.co.uk/board/showthread.php?t=15073 it doesn't just apply to goods bought over the net but also has some interesting info about your rights period.
HTH