- Close all warning and error message windows and return to the main menu.
- Open the Fax menu
- Mark the '*' Button with a pencil, or just remember where it is. Do not press it. It will disappear in the next step.
- Return to the main menu.
- Open the printer so you can see the toners.
- Press and hold the hidden '*' button for 7 seconds.
- A toner reset menu appears. Reset the toner in question.
- Choose K=black, C=cyan, M=magenta, Y=yellow, STR=starter (small), STD=standard, HC=high capacity.
- Close the printer and return to the main menu.
See also https://www.timoschindler.de/brother-mfc-9140cdn-toner-resetten/
See also https://www.timoschindler.de/brother-mfc-9140cdn-toner-resetten/