Also, some models of laser print engines, like most inkjet printers, communicate with “chips” or fuses inside the toner cartridge which indicate that the cartridge is exhausted, whether or not it actually is. Some research is usually required to determine the feasibility of refilling a particular cartridge.
In general, DIY refilling requires opening a cartridge designed not to be opened, refilling it with toner, and capping the opening that was made to gain access. Some cartridges can be disassembled; others require a hole to be drilled or melted.
Toners vary in composition; manufacturers make toner designed for their own printers, and a generic toner may not work as well. In some cases there may be separate additions of toner and “developer”. Refilling kits include printer-specific instructions.