I just recently had this problem. 3 hours on 3 separate chats with Comcast produced no results. I figured since they were seeing a signal going to the box, the problem must be after the box. So I bought a new piece of coax cable and reconnected from the box to my TV and voila, problem fixed. I believe Comcast provided that piece of coax cable with the box which was only about one year old. If you are using HDMI cable, try replacing it.
... View more