A common business model for inkjet printers involves selling the actual printer at or below production cost, while dramatically marking up the price of the (proprietary) ink cartridges (a profit model called "Freebie marketing"). Most current inkjet printers attempt to enforce this product tying by anticompetitive measures such as microchips in the cartridges to hinder the use of third-party or refilled ink cartridges. The microchips monitor usage and report the ink remaining to the printer. Some manufacturers also impose "expiration dates". When the chip reports that the cartridge is empty (or out of date) the printer stops printing. Even if the cartridge is refilled, the microchip will indicate to the printer that the cartridge is depleted. For many models (especially from Canon), the 'empty' status can be overridden by entering a 'service code' (or sometimes simply by pressing the 'start' button again). For some printers, special circuit "flashers" are available that reset the quantity of remaining ink to the maximum.