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I have a machine running Ubuntu 20.04 which was connected to a 4K display (Samsung), and is now connected to a different 4K display (Dell). Since rebooting the machine, just before it gets to the login screen, the display goes blank and the display notifies me

No HDMI 1 Cable. The display will go into standby mode

Why would a different display cause this and how do I fix it?

Note : I've just inserted an Ubuntu Live USB device. Afer checking the disks I get the same thing, just a blank screen and the message:

No HDMI 1 Cable. The display will go into standby mode.

Note: As requested, the Dell monitor model is S3221QS

Matthew Layton
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1 Answers1

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Diagnosis

This problem will require back-and-forth interaction (much in the same way as in forums).

Just a few general questions:

  1. Please post the complete models of your monitors.
  2. Are you using the same HDMI cable to test both monitors?
  3. Did you try connecting both monitors at the same time? If not, please do so. Try exchanging them as primary/secondary, and switching the ports you connect them to. Report what you get.
  4. Do you have Windows in your system, to test the same as you are using now but with Win? Please do so if you can. Report what you get.
  5. "I tried this in safe mode"... Is it this answer? Which of the two alternatives? (It usually helps a lot if you give as precise info as possible).

In addition to the suggestions above, consider a few others, and diagnostic commands. Please copy in your OP each command with the full output you get.

  1. Post the output of

    2.1. xrandr (with one and two monitors connected if that makes a difference in functioning).

    2.2. sudo lshw -C display (with one and two monitors connected if that makes a difference in functioning).

    2.3. uname -a.

    2.4. dpkg -l | grep nvidia-driver.

Possible solutions

In addition to the suggestions above, consider a few others. Please report in your OP what you get.

  1. Try the sequence below, you would need the other monitor connected for this (source, albeit for a somewhat different problem).

    1. Turn off the PC.
    2. Disconnect the HDMI cable.
    3. Turn on the PC (without HDMI).
    4. Log in.
    5. Reconnect the HDMI cable.

    See also other answers in HDMI port not detected on Ubuntu 18.04.

  2. This would be worth trying if everything else fails. Reinstalling with (source)

    $ sudo apt-get install --reinstall gdm3 ubuntu-desktop

    or probably less invasive

    $ sudo dpkg-reconfigure gdm3
sancho.s ReinstateMonicaCellio
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