Which ink does the HP OfficeJet Pro 6978 use?

The OfficeJet Pro 6978 takes the HP 902 family, four separate cartridges: an HP 902 or 902XL black plus 902XL cyan, magenta and yellow.

Black is the expensive side on this machine, which surprises people who assume colour is what costs. A 902XL black is rated 825 pages for $38.00, or 4.61c a page, while each 902XL colour is 825 pages for $30.00, or 3.64c. Black therefore costs about 27 percent more per page than any one of the colours, so the standard advice about printing everything in greyscale moves your work onto the dearer cartridge rather than off it. Between the sizes the decision is easy: the standard 902 black is 300 pages for $18.00, or 6.00c, so the XL takes 23 percent off. A full XL set is $128.00.

Inside HP's own range that is mid table rather than good. The OfficeJet Pro 7740 in our catalogue runs an HP 952XL black at 2.25c, about half what this printer pays, and among the 82 inkjet blacks we price the 902XL sits 26th from the cheapest against the 952XL's 7th. Both are OfficeJet Pro machines and both are aimed at the same kind of small office, so the gap is cartridge capacity rather than class: the 952XL simply holds more.

HP applies Dynamic Security to the 6978, which means the firmware can reject a non HP chip, and the rejection typically arrives with a firmware update rather than with the cartridge. A compatible 902XL set that has printed happily for months is exactly the situation the scheme is designed to end. If you plan to buy third party, settle your position on automatic updates first. Our ink DRM tracker covers how the scheme has spread across HP's range.

When something goes wrong, work out which kind of fault you have. Ink System Failure and similar carriage errors normally clear with a hard reset, meaning the printer switched off at its own button, unplugged at the wall for a minute and plugged back in. A single cartridge reported as damaged is most often contacts, so wipe the copper pads and the carriage contacts with a dry lint free cloth and reseat firmly. Colour missing from an otherwise clean page is a printhead question and starts with a nozzle check, because on this printer the head is built into the machine and a cartridge swap will not fix it.