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I have an HP EliteBook 840 G7 Notebook, with Linux Mint 20.3.

My goal is to connect two more monitors to the machine.

I have a monitor connected via HDMI port, and a monitor connected via USB-C port.

Both monitors at the moment work if connected individually, but if I connect both, the one on HDMI takes over, and no signal goes to the one connected via USB-C.

The strange part is that I have been able to work with three monitors for some time, up until (several months ago) I have upgraded the system (from 20.1 to 20.2, I think) ad I lost this ability.

After reading this answer I have tried to change settings in the bios (allocated bigger video memory, and "enabled high resolution mode" (something involving USB devices in a dock, and Gigabit NIC), but I had no results.

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I solved the problem changing the resolution!

I lowered the resolution of the two connected and working monitors (built in and hdmi) to a minimal value. Then I reconnected the one on usb-c, and "detect monitors" in the "Monitor preferences" panel. And the third monitor suddenly appeared in the "Monitor Preferences" window.

Then, step by step, I increased the resolutions of all of them, so to find the maximum and/or best configuration.

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  • So, the problem is that the video card can't handle thee monitors with large resolution? I had this problem before: I could connect three monitors but only two of them could be active at a time. If you use LCD panels, using native resolution gives better results. Commented Dec 31, 2022 at 13:09
  • No, the resolution can be high on all of the monitors. Over the last weeks I have had unpredictable behavior. At the end of the day, the rule of thumb is "try again and again, plugging, unplugging, and changing resolution, and at some point it will work". However, I couldn't have it stable. Meaning you need to be ready to restart all over again, next time you restart the computer.
    – Fabio
    Commented Jan 9, 2023 at 12:14
  • I probably know the solution. Check my answer here and/or tell me the output of xrandr --verbose | egrep 'current|pref' when all monitors work as expected. I can create a step-by-step guide to go back into this configuration again. Commented Feb 24, 2023 at 2:02
  • Thank you for the reply @RobertSiemer ! At the moment I had to move one of the monitors, so I can't replicate the situation. I will follow up as soon as I can. Thanks again.
    – Fabio
    Commented Feb 26, 2023 at 9:02

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