That sort of tells us the trouble is from the HDMI inputs to the mainboard.
Since you know your set, my thought would be to swap the HDMI input board. If that's on the mainboard then that's suspect here.
Mind you this is just me sharing what I think. The real way is to get the service manual and follow its diagnostic tree.
This is concerning my LG television model 47LV4400-UA. It powers on (red standby light turns blue) and the menu appears on the screen, but there is no picture. “No signal” message on all inputs.
I tried all the HDMI ports and switching HDMI cables for the cable box. With the 2nd cable, the screen went solid pink but I think that was a faulty cable I had around. With the 3rd cable, a message “HDCP authorization failure. hdmi port is disabled” appeared over the picture, then “No Signal” returned after 10 seconds. I plugged in the original HDMI cable and reset the cable box. I again received the “HDMI port disabled” message as the cable box booted up, followed by the “No Signal” screen.
I also tried streaming video using my Chromecast and audio would play through the TV, but no picture. Reset the Chromecast, same problem. The next day, no audio or video.
Any thoughts on the possible issue? I unfortunately do not have another TV to test the devices on
Thanks for your time!