For the second time in as many HP printers, I ad a problem with the printer not printing from the black ink cartridge. I thought it was damaged when a sheet paper got jammed a few days ago. I scoured the interwebs for solutions and found this:
This worked great for my HP 5520. I had to put a damp paper towel in 5-6 times. I also unplugged my printer and left a damp paper towel under the printhead overnight. My prints look so great now. I am thinking this cleaning needs to be done regularly. It saved me having to go buy a printer.