If you receive the following error, what has most likely happened is that there has been a connectivity issue with the Xbox Live servers whilst the code was being applied to your account.
Sorry, there's a problem with the Xbox service. Try again later. For help with this problem, go to www.xbox.com/errorhelp.
Status code: 801613C9
If you try to enter the code again, you may receive a message saying that the code has already been redeemed.
Usually, when this error is displayed, the code has already been applied to your account. In which case, the best course of action is to reboot your console. You should then find that the code is showing on your account.