I would simply like to state here for the world that if your TV displays the specific text "No Signal" upon correctly plugging in your cable box to your TV via HDMI (and you have verified that these connections are tight and the Source input is correct), this is very likely a service problem and not a technical problem with your box/cord (though if you can, you should test a different HDMI cord and/or box to verify). Specifically, your HD service is not activated properly. "No Signal" implies that the TV knows your box is there, but the box is not sending information to it.
I have spent the past month dealing with this issue and have been led through so many hoops incorrectly, wasting a lot of my time time.
Specific problem: the cable worked correctly via non-HD component input, but for the HDMI input, the TV says "No Signal". We have an HDDVR Comcast box and a Samsung UN40EH5300 Smart TV, though based upon a couple other posts on the internet with a similar problem that had other TVs, the TV type does not seem to matter here (and I nearly returned my TV thinking it was the problem).
Course of Action: First we made sure the Source input was chosen correctly, the cord was plugged in correctly and snugly, and we tried every HDMI input port on the TV. Then, we attempted to power cycle the box and the TV. We also verified that both a DVD player and a computer both displayed correctly via the HDMI input on the TV, indicating that the HDMI ports do work. We also tried two other HDMI cables. Then we were told by a Comcast service rep that the HD service should be working and so it's a problem with the HDMI cable and/or box and so picked up new box/cable from Comcast Service Center (20 miles away). Then we spent 1 hour in a web chat with a service rep, who made us do many of these same things again before concluding that a technician must be sent (5 days later).
The moment the technician arrived, he saw "No Signal" and stated openly that this should be a service problem and not a technical problem -- that our account simply does not have HD service activated. This obviously confused me since we were told by Comcast that the HD service is activated. He tried a brand new cable box anyways with no change, and so concluded again that it should be either a service problem or a problem with the TV.
To check once and for all if it is a service problem, I called Comcast customer service directly. They found out that our HD service was indeed not activated properly (!?! this had been verified before as supposedly working correctly). I made the rep activate the HD service while on the phone and -- magic! -- it worked right away via HDMI. It took a mere 5 minutes.
So if you receive "No Signal" you should first check your Source input, HDMI cord connections, and, if possible, try a different HDMI cord first and a different input (e.g. DVD player) via HDMI to make sure your TV reads HDMI correctly. However, if this still doesn't work, make sure your HD service is activated correctly, and if needed do so on the phone with a rep on the line before resorting to a technician or other long processes (e.g. returning your TV). It is apparent that the Comcast account folks are eager to tell you its a technical problem and pass you off, potentially wasting a lot of your time.
I hope this helps save others a lot of time.
Central New Jersey
Thank you very much for taking the time to document this. I had exactly the same issue and came across your solution after a live chat and two conversations with "techs". After reading your post I called back and had hdmi video output within 5 minutes allowing me to cancel my service call appointment.
As the good man said above, if you have a blank screen over hdmi and have ruled out components and cables, call comcast and tell them to double check that your "HD service is configured correctly",.........because it isn't.
Thanks again Maltaj!
I would like to also say thank for you making this post. I intially thought I had a bad box. Took it back and after activation of a 2nd box I had the same exact issue. After phonecalls and no help something said check the forums. Stumbled across your post just used the chat a lady name Jennifer was able to activate the HD onto my new box. Comcast needs to have better training to these folks that they are outsourcing to as they were no help in which they kept referring a techinican was needed!
I am having a similar problem and I called comcast and they claim it is a technical issue and are sending someone to check it. I will call Comcast again. I tried replacing the HDMI cable.
Now I know to make sure your HD service is activated correctly, and if needed do so on the phone with a rep on the line before resorting to a technician or other long processes (e.g. returning your TV). It is apparent that the Comcast account folks are eager to tell you its a technical problem and pass you off, potentially wasting a lot of your time. THANKS for taking the time to explain the problem.
Praise be Dan for posting this! I'd like also to add if no other factor in your service has changed other than you had a signal on the last day of your billing cycle and the next day of the new billing month cycle you turn on your TV and get the 'No Signal', keep pointing to that as a possible root cause. Otherwise, first-level technical support will take you down the rabbit hole described by Dan and others on this thread. My incident today of no longer receiving a signal at the start of my current billing cycle happened to be I wasn't being charged a $10 "HD technology fee", despite receiving an HD signal for years. Once I called Billing and had the fee added, I too was sent to the Service Center to get a new set top box, only to have the rep wonder why I was swapping one HD box for another. She did it anyway, but I firmly believe it was the proper transmission of the activation signal for the swapped set top box was the solution. Now I receive Streampix when I didn't before even though it was a part of a service change I had made months before today's problem, yet didn't bother to report because of the ordeal it can be. As someone who has been in technology as a career for decades, Comcast's systems, particularly their billing system, must be profoundly messed up.
I get the same message but intermittently. So it can't be the cable. I'll be watching a show live or on DVR and lose picture but have sound other times lose both. They had me swap out the box same problem.
I'll call them again and see if they can boost the HD signal or reset it.
Thank you so much for taking the time to post this. I had the exact same problem and followed your instructions and it worked!!! Been without cable for 1 week, not the end of the world, but I am paying for a service and expect to be able to watch TV.