A colleague of mine came into work to find that his dual monitor setup on Ubuntu 12.04.3 was no longer working correctly. The main monitor (connected via HDMI) was being detected okay, but the secondary monitor (connected via a VGA cable) was appearing as “Unknown” and limited to a resolution of 800×600. He was also seeing errors about the “EDID checksum” in his logs.
After a bit of searching on the internet we tried various ways to fix the problem, from the obvious re-seating the VGA cable to the less obvious installation of “mesa-utils”, but to no avail.
The thing that finally worked – as is often the case with computers – was to turn it off and on again. Obviously that was one of the first things we’d tried: the computer had been shut down, the monitor turned off, then monitor followed by computer switched on again. But that alone wasn’t enough to fix the issue.
What was actually required was to completely unplug the mains lead from the monitor. Simply pressing the power button on the front isn’t enough for the monitor’s processor to do a full “reboot”. Pulling the power cable (and VGA lead for good measure), leaving it for 30 seconds, then plugging both back in fixed the issue immediately. Ubuntu could recognise the monitor once again, and it was automatically set to its intrinsic resolution of 1280×1024.
So if you find yourself with an EDID checksum error, before you get too deep into the world of xrandr or Xorg.conf files, try completely removing the power from the monitor. It just might save you a lot of time and effort!