When the toner life end message comes up, there are 2 things that need to be done to keep printing. The windows on each side of the cartridge need to be blocked (with electric tape or something similar). And the flag gear on the cartridge needs to be reset. Here is how to do that:
1. Orient cartridge so that you are looking at the end of the cartridge with the 2 phillips head screws, one at the top right, one at the bottom left. You should also see part of the gray flag gear through an opening in the end cap.
2. Remove screws and the end cap they were holding in place (one or 2 gears may fall out when you do this, but they should be simple to pop back into place).
3. Note attachment point of the spring over flag gear, then temporarily remove it so you can reset the gear.
4. Remove the flag gear and reinstall it so that its teeth are just beginning to mesh with the adjoining gear.
5. Reinstall the spring.
6. Reinstall the end cap.
You can also reset the cartridge from the printer. But this will only temporarily override the toner end of life message and allow you to print a few more pages. Here are the instructions for that:
1. With power on, open the toner access main door (You will get a "door open" message in the LCD.)
2. Hit the "Clear/Back" button and you go right to the "Toner Reset Menu"
3. Using the up & down "Search" arrow buttons, you can then scroll through reset options for each of the printer's  toner cartridges.
Code: B-TNR-H = black, C-TNR-H = cyan; M-TNR-H = magenta and Y-TNR-H = yellow. The H is for high yield.
4. Highlight the toner you want to reset and hit the OK button.
5. Hit the "Clear/Back" button to get out of the menu, close door.