cartridge problem

Hi,
i have a small problem using my printer.
i own a HP Deskjet 840C printer which i bought a couple of years back. i used to take both color and b/w prints. (b/w frequently, color print rarely).
when my black and white cartridge ink got over, i replaced it with a new one. i did not replace my color cartridge and left the old one as such hoping to use the printer only for taking gray scale(ie b/w) prints.
i did not use my color cartridge much, but still its ink too got over? why?
i did not buy a new color cartridge, but since the manual specified that the cartridge cradle should have both cartridges (even if one of them is empty) tp print, i left the color cartridge as such. now when i use my printer, i get printouts in which the figures have light shades of other colors, even if the original figure is a black and white image and i choose the gray-scale option when printing.
can you tell me what may be wrong and how to proceed?

One possibility might be if your printer drivers have become corrupted. Try reinstalling the software that came with the printer.

Tom

If not the drivers it could just be a dirty printer.

It could be that the coloured inks have leaked out during down time or automatic head cleaning and the print head cradle and print head itself have become contaminated with semi dried coloured inks.

It is difficult to clean the inside of a printer without taking off the cover. (If you do be cautious you can snap internal ribbon cables - but can buy replacements parts from HP website.) But try dabbing where the Print Head and cartridges rest at the right side of the machine using a damp clean soft cloth when the head/cartridges etc. are in the centre of the printer. If it comes up covered in ink - you need to get cleaning.