My cable box displays SISt basically all the time. It may resolve itself for a short amount of time but it always returns. What is this code related to?
I suggest you pull the power plug from your X1 box and plug it back in; let us know what happens.
I have unplugged on multiple occasions and it always reverts back to the same code within 24 hours.
if you are sure it is a connected outlet, swap the box at your local xfinity store.
Swaps are free. Be sure to use the new power brick; they are sometimes the cause of issues.