This video shows you how to get your Brother DCP 9020CDW printer working again without shelling out a fortune for new toner cartridges. If you just want to know how to do do it just watch the video. Note that this worked for my Brother DCP 9020CDW printer but it may work for other similar Brother models too. It will certainly work for any multifunction printer that uses the TN-241 / TN-245 cartridges but the control panel on your printer may have different buttons/options to press - they do all use a similar technique to reset the page count.
I found that the default toner empty setting was nearly always false and that lots of toner was always left. Use this setting to override the default setting and use up all of your toner. After all you have paid for it !