The card & box are paired on your account, so you cant put the card from one box into another. If the spare box has a card, then that card will only work with that box. The boxes are rented, so there is no charge to replace a faulty box, if indeed the problem is the box in the first place & not a network wiring issue.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV+), Talk Unlim Telco, VIVID 100, Virgin PAYG Mobile