[Cura is not turning on fan during print]

I think I figured it out.

 

Cura 4.0.0 now has a new feature where you can select different number of cooling fans.

I looked at the gcode exported from cura 3.4.1 and the gcode from cura 4.0.0. I noticed the M106 command was exported as M106 S255 P1. I compared this to the gcode exported from cura 3.4.1 which just had M106 S255. I first tried to modify the gcode to have M106 S255 P0, AND THAT WORKED Later in my investigation I noticed in the "Machine Settings" under "Extruder 1", there is a new setting called "Cooling Fan Number", by default it was set to 1, hence the "P1" reference in the gcode.

Change the option under "Cooling Fan Number" from 1 to 0. 

 

After that I saw cura exported the gcode as before M106 S255. I popped in sd card and it seemed to solve my issue. I am sure maybe in the Marlin configuration you could change the fan number as well but this seemed to fix my issue.

 

Hope this helps! 

[Cura is not turning on fan during print]

Yes same here. I can confirm that the issue is with cura 4.0.0 I think, going back to cura 3.4.1 no issues but with cura 4.0.0 I get no can as well.