brother printers

[didappointed]

I have 2 printers, brother DCP120C and MC660CN. Both have devepoled the same "fault". They show that the yellow cartridge needs replacing. I have replaced them on both machines, even changed them again in case they could have been dud ones, although they are genuine Brother cartridges. I am left with 2 printers and neither works. I have done are the checks possible they show in the problem pages to no avail. They still show "cartridge empty". Has anyone experience with these printers and can advise me what to do next without having to call in repair people?