HP 2840 is printing light even after putting in a new cartridge

You need a supply of ink in both cartridges in order to print. Even if you are only printing black ink and there is a black ink cartridge with ink in it, your printer will not print without the color catridge installed and it has to have a supply of ink in it. I wish I had better news for you, but unfortunately I don't.

It seems simple enough, but this is one of the most effective ways of fixing printer cartridge error. Mostly, printer cartridge error occurs when the printer head is unable to identify the cartridge properly. And the basic reason behind this is that the cartridge is improperly attached. To reattach the cartridge, turn on the lid of the printer and then remove the cartridge from the printer head. Check the cartridge to make sure that there are no scratches or other damaged parts that can contribute to a printer cartridge error report. Take note that the cartridge should not be left open for more than 30 minutes; otherwise the ink will start to dry up and the cartridge will get clogged.

Follow each of these steps until the problem is resolved:

Print a printer self-test by holding the power button down with a pencil and opening and closing the top cover four times.
Check the nozzle self-test pattern in the self-test print out. If one or more of the solid rectangular areas are missing, an electrical contact problem may be present.
Remove and then reinsert the print cartridge(s) into the printer.
Print the document.
If the problem persists, follow the steps listed above for cleaning the print cartridges and print cartridge cradle.
Try printing again. If the problem continues, replace the print cartridges.

http://www.fixya.com/support/t5717191-hp_deskjet_6540_printer_yellow_light

try taking the cartridge out and cleaning the connection then putting the cartridge back in

Yes, sorry to say. The system uses all carts even to print in black.

All cartridges are new...report shows 0 pages for yellow left yet all others added at the same time show thousands of pages left. Everything printed is in black with no color. BTW: I have 3,000 pages left with black cartridge and the display says I need black.

What should I do?

you need to print only in black if that is all the ink you have, and also you need to remove the color cartridge as it still registers in the printer as able to be used, although it is empty or near so. Did you remove the tape over the print head fron the new ink cartridge? Do a print head alignment, or use the toolbox icon to tell the printer that you just replaced the cartridge. Some HP models chip the ink cartridges so even a refill wont register as a full tank and the printer wont print. chip needs to be reset (difficult) or just replaced with a new cartridge.

Is the color cartridge new and do you know how to check the gauge in your control panel for the ink levels?

If one of the color cartridges runs out the machine will switch to black only mode. However, I have found no way to force it to continue printing in color mode when any one of the cartridges runs out.