Added new ink cartridge and now it says its empty

No.
The Color cartridges must stay in the machine or it will not feed the paper through.
It is a colour printer, and will always need cartridges. Even if set up for mono printing, there mat still be a tiny amount of colour toner added to the page. (Model dependant)

The Fax-2480c does not use toner cartridges.
This is an inkjet and so uses ink cartridges.

There are 4 cartridges, black, yellow, cyan (blue) and magenta (red). If any of these cartridges are empty, you need replace them.

This machine takes LC-51 cartridges.
You're probably out of warranty so jsut get refills... it's cheaper

It needs ink in both the color and the black in order to work properly. It is just something HP does to make sure that the printer will work properly.

MAYBE YOU ADDED A BLACK INK CARTRIDGE RATHER THAN THE COLOR ONE ? OR MAYBE YOU HAVE THE COLOR CARTRIDGE PLUGGED INTO THE BLACK INK ONE ? DONT KNOW IF THIS WILL HELP BUT THOSE WOULD BE THE FIRST FEW THINGS I WOULD CHECK. I ALWAYS SEEM TO MISS THE OBVIOUS WHEN SOMETHING ISNT WORKING FOR ME

If it is an HP or Brother the printer will only work with full cartridges both color and black. You must have ink in all cartridges

I have the same problem. I just discovered that if I turn OFF the power before I open the cover to check a cartridge it does not ask me if I just replaced it. I hope it was not just a fluke, otherwise I'll try unplugging the power next time, too. As for the "Can't print with an empty cartridge" problem; IE: Red is empty but you want to print with black; it is either that engineers don't know what they are doing at Brother or they are lazy or the managers are GREEDY. I think it is the last one. That is why I try NOT to buy "Genuine" Brother Ink cartridges. I used to refill them myself when I could find the ink, now I buy 3rd party cartridges. I pay $6.00 each for black and $7 each for color at a "Toner / Ink" shop about 1/2 mile from my office.

Even with out the Black cartridge installed the X1240 should use the color cart to make a "composite" black. I would suggest calling Technical support to troubleshoot that issue ad get you to test the cart itself

this is what i did as a last resort after messing with this stupid printer all day and changing cartridges and getting a no cartridge message.
i took all of the ink cartridges out and took a pair of pliers and pulled out what i think must be the sensors. they are black plastic pieces toward the top part of the ink area. then i put the cartridges back in and i have been printing ever since. this was my last resort before i was prepared to throw in the towel so that was why i went at it with the pliers!

cucujo, you were not as lucky as lovetofish! Sooner or later the printer will win.
The money for printer manufactures is not in the printer, it's in the ink refills. With this in mind many printer Mfg's have added small chips to their ink cartridges. These chips track the amount of times the printer prints. The software knows you are adding ink to the cartridges. Many people find they can get around this by keeping their old cartridges & refilling. Some HP users keep as many old cartridges as three. Rotating ink cartridges. This seems to work for most printers. Suggest you try the same. Continue to fill you're ink cartridge but replace with a new cartridge. When it empties replace with another new. The next three will be re-filled ink cartridges. Always refill as soon as possible, never turn ink cartridges on their sides. Keep refilled cartridges in an airless zip-lock bag, in a cool & dark place when storing.

Mike

i ended up putting a color and black cartridge in and everything is fine.