Printers are programmed to tell you to replace a toner cartridge before it is necessary based on counters. There is a virtual counter in the machine itself. I have seen a lot of videos on how to reset this, but that doesn't always work. There is also a physical gear counter on the toner cartridge. So even if you are running out of toner, you can buy and replace just the toner and then reset the toner cartridge. This video is for my Brother HL - L2395DW, but I'm sure similar situations occur with other cartridges.