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Hello, if you lift the top cover there should be a little label with the correct ink cartridge numbers that you need to buy. I've got a Deskjet 845c, which is similar to your 840c, and the label shows 17 colour; 15 black. I suspect yours will be the same. No others will work as the printer was designed for those particular cartridges. They are still readily available in the shops or online. If you're in the UK try www.inkrite.com
Go to www.hp.com > Downloads > Put HP Deskjet 845C in as model > Select Operating System (Windows XP, Vista or 7)> Download driver (select basic or complete) > Disconnect USB connector to printer > Double click on Printer driver to begin installation > Connect USB from Printer when told to do so during the Printer installation. Hope this helps.
i dont know if anyone has logged this, but i would think that logically the lights would continue to blink until it detects the right amount of ink density and If they are chipped that the chip registers correctly. Ttry installing just one of the cartridges ( the cheaper one)