Bro here is the algorithm...
So, here's the code i used: 304239734848
The serial number is so composed: LNNNNNN YWWFF
L = production line number
NNNNNN = number of the xbox on that week
Y = pruduction year last digit
WW = week of the year
FF = factory code
Therefor the xbox one related to my serial was in the 3rd line of production, was the 042397° produced in the year 2013 on the 48th week of the year with factory number 48.
Now I don't know any other factory code because i saw a limited number of serials, all of which had 48 as factory code.
To have more chances to be the first to claim the console you should use a quite recent week, as i did week 48 which is somewhere around november.
There's one important rule: all the digits summed up must be equal to 55 !
3+0+4+2+3+9+7+3+4+8+4+8 = 55
Now you're able to make your own serial code by changing the Ns and L with random numbers ( remembering the rule ) and try SEing if you want.
I'll try later today because all I have available is repair or replacement paying €99 and sending the malfunctioning console to them... I'll try and chat with an agent and see if I can get something done.
All credit to @Hustled