There is a little window on the cartridges the printer shines a light through to detect the toner. When it can see the light you get out-of-toner errors. If you mark over it with a sharpie, it will print again. I'll be the judge of when it's out of toner based on how faded the print is. As infrequently as I print this trick can make a cartridge last months longer.
I used to use a common hole punch to make a little disc from black electrical or black gaffers tape and use that to cover the toner windows because I found that Sharpie just wasn't opaque enough in all cases, especially with my Brother color laser.
And then I discovered the paint pens my wife uses for rock painting. Now, those really work.
