My printer (a Brother HL-5170DN) declares that it ran out of toner (it uses a TN-3030 cartridge) while printing quality is still perfect. This 'problem' is indicated by the continuously red 'Status' light, while the toner light is also on (continuously). Moreover, it refuses to print.

Inspired by this site, I decided to try something similar and it works for me.

Here are the steps (of course, you attempt these at your own risk!):

1. There are 2 little circular (less than 1cm diameter) holes on the opposite sides of the cartridge (bottom left and right). They look like glass windows with some black stuff behind. Stick a white paper (I used small adhesive stickers) in front of each of them.
2. Have the printer on with the lid (where the toner goes) open (and the toner cartridge installed, obviously). The red light should be on (continuously).
3. Push the GO button until all lights (toner, drum, paper, status) have all gone on and off again.
4. Close the lid. The green light should come on.