Toner messages
Toner Low is a warning and you can keep printing. Replace Toner stops the machine. Before you buy anything, take the cartridge out and rock it side to side, because the sensor often calls it empty while there's usable toner sitting in one end. If you've fitted a third-party cartridge and the printer refuses to see it at all, that's the chip being rejected rather than a fault, and a genuine cartridge will tell you within a minute which one you're dealing with.