My PC is 5 yrs old, ok 6, running XP. I had an old HP 720C printer that stopped printing yellow. Changed the cartridge (and thereby the printhead) but still no yellow. Must have tried 3 or 4 different carts. Changed the parallel lead, still no yellow. emailed HP support but the time would have been better spent trying to teach the dog to play chess. Bought another HP printer off ebay, this time a 890C (I've got a free supply of the cartridges you SEE) BUT IT STILL WON'T PRINT YELLOW. Arrggh. Please if anybody knows anything about this LET me know, it's keeping me awake nights.Sounds like possibly the port DATA port is defective for the yellow input.  Do you have another COMPUTER that you could test the printer on?

Alan <><  I have a laptop but it has no parallel port. If that's the answer then I'll havt to get me a hybrid cable. Might I have dislodged something in the PC? I OPEN it up a couple of times a year and give it a good blow out with compressed air.Perhaps you could simply try a new cable.  They should be pretty cheap.