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My Lexmark E323 laser-jet printer was indicating that the toner level was low. I wanted to find out how it knows that, and if I could fool it to think there was still plenty of toner.
cartridge chips
Printers don't actually measure how much toner (or ink) there is left in the cartridge. This would be too complicated, expensive, and isn't actually needed. Instead you just need to store the information of how much toner there was to begin with, and update the value with each print, since you can calculate how much a print will cost. This information (the toner level) is often stored in a chip embedded in the cartridge.
Some printers allow you to still print even when the cartridge is theoretical empty. This is the ideal case if you want to refill the cartridge yourself (toner is not very expensive). Other printers will not allow you to do so, and in this case you need a new cartridge, a new chip, or a chip reseter. You can already find counterfeit chips for most printers (when required).
I could refill the cartridge for my printer without issues, but at some point or another you will still need to replace the cartridge since the other parts (like the OPC drum) also degrade with usage and will cause poor quality prints. At that point it's probably worth to buy a new printer since the main cost is the cartridge itself. Still, I wanted to find out how the chip works.