I had the luxury a couple of days ago of doing this, and reading the manual that came with the cable box on how to program your TV and any auxillary device (that includes the codes needed to enter into it to match your other devices), it turned out that the manual was wrong.
It explains in the step-by-step instructions to enter one of the 4 digit codes, and after entering the 4th digit, the light would flash twice. Mine never flashed twice, even on the 100th time I entered a code. It seemed to need 5 digits. After playing around some I figured out that it needed a 1 entered before any code (for the TV). So for example, 4056 should've been 14056. I also figured out that at least for my auxillary devices, the number 2 needed to go first (not 1).
All of this could've been avoided however if I'd of had the correct manual with the correct codes (thanks Comcast), which I later found here. That's for the remote that looks like this:
So if you find you're remote isn't taking any codes, give the codes in that manual a try, or try adding a 1 or a 2 in front of the code.
In the end however, it really didn't matter ... that's the thing about these universal remotes ... there's always that ONE button on the original remote that is still needed, and the universal remote doesn't do it, even with the correct code for your device. Therefore you're still left with 7 remote's laying around ya.