Setting the Coolbits in 20.04 without blowing X

From my understanding, coolbits option should be in Section "Device" and not Section "OutputClass".

For only manual GPU fan control, you can set coolbits to 4. 28 also works, as it's 16+8+4 (each being a different bit in binary), giving you two more sets of options as well (I think one is overvoltage (16), and the other is to change performance settings per clock (8).

You're on the right track. It's Option "Coolbits" "13". Also, my conf file is xorg.conf at /ect/x11 (you can generate one with nvidia-xconfig). It looks like this:

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 455.28

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 440.82

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 435.21

Section "ServerLayout"

    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
EndSection

Section "Files"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"

    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "Microstep MSI G241"
    HorizSync       180.0 - 180.0
    VertRefresh     48.0 - 144.0
    Option         "DPMS"
EndSection

Section "Device"

    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 1070"
    Option         "Coolbits" "13"
EndSection

Section "Screen"

    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "TripleBuffer" "On"
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-6"
    Option         "metamodes" "DP-4: 1920x1080_144 +0+0 {AllowGSYNCCompatible=On}, HDMI-0: 1920x1080_60 +1920+0"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

TLDR:

In /etc/X11/Xwrapper.config add:

allowed_users = anybody
needs_root_rights = yes

and change permissions:

chmod 2644 /etc/X11/Xwrapper.config

I got this working in a multi GPU config with 470.63.01 drivers. I thought that the problem was with Coolbits, but it actually was the Xwrapper.config (by the way, I set the Coolbits in /etc/X11/xorg.conf).

I'm not sure why the command:

sudo nvidia-xconfig -a --cool-bits=16 --allow-empty-initial-configuration

breaks your X. For me it worked, and X is behaving normally so far. I'm using the default GNOME and multiple desktops. I cannot assist you with this, however, because it was not the problem. This command added a bunch of lines to the /etc/X11/xorg.conf file, containing 3 entries for each GPU, that look like this:

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "Unknown"
    Option         "DPMS"
EndSection

...

Section "Device"
    Identifier     "Device1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "NVIDIA GeForce xxxxxxx"
    BusID          "PCI:x:0:0"
EndSection

...

Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "AllowEmptyInitialConfiguration" "True"
    Option         "Coolbits" "16"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Even after applying the command, the fan setting won't work unless the /etc/X11/Xwrapper.config file is edited. I had to add the following lines:

allowed_users = anybody
needs_root_rights = yes

Then save it and change the permissions:

chmod 2644 /etc/X11/Xwrapper.config

Adding these lines and rebooting did the trick for me. Without them, nvidia-settings just fails. I found this solution here.