Ubuntu – Display not detected on a Thinkpad T420

Try setting a lower frequency and/or updating drivers. I've used to have this problem with old (up to this July) drivers and the workaround was to set an external monitor to 60 Hz (by default it was set up to 75 Hz). While lowering resolution as you do is not a pleasant solution, lowering frequency usually makes no visible difference unless you use a CRT display.

If you want to update your graphics drivers, open a terminal console and type

sudo add-apt-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get upgrade

After restarting the computer, your system should be running the current graphics drivers. I hope this helps

• Is it safe to install updated Intel drivers from the x-swat PPA?