HP 4si

Hi,
When I've worked on printers with this problem in the past it usually comes from the high voltage power supply going bad. There is also a low voltage power supply that runs everything except the actual printing so your printer will still operate without any errors but the print quality will go bad. The high voltage power supply is located under the bottom of the printer and it's too hard to replace. I can't advertise for any place on this site, but if you type in the part that you need into a search engine and look for parts on everyone's favorite bidding web site you should find the parts you need. Good luck.