I’ve had many inkjet print heads that eventually got dried up and started streaking when printing. These supposedly delicate parts consist of a very fine, narrow channel for ink of each color. The extremely narrow width allows them to position ink precisely, but it also means they can clog easily with dried ink.
The solution for clogged ink heads is pretty simple: Rinse with hot, hot water. Boiling water, in fact. I poured a small cup slowly over the print heads for about ten seconds, dried it off with a towel, and stuck it back into the printer. PRINTS LIKE NEW!
Many sites advise treating the print head much more carefully: make measured-out solutions of ammonia, soak a paper towel, and leave the print head resting on the towel overnight. But soaking for hours has never worked for me. Lesson learned: The print head is remarkably resilient.
I’ve tried this now with two printers: a Canon i560s and a Canon iP1800. Worked perfectly both times. The iP1800 has cartridges with built-in print heads, so I worried that water would enter and dilute the just-refilled cartridges. The cartridge would just be junk if I couldn’t get it working, though, so I gave it the scalding-hot treatment anyway. It worked!