I have lately run into this problem and have found the solution to be much
simpler than is advised below by the specialists.
I have simply opened up the hot water at the sink to the full at the hottest
possible, and let the water run into both openings.
The ink will run out, but do not stop when the ink stops running.
Continue with the water for a few minutes more until a second run-out of ink
occurs. Stop only when after several minutes no more ink comes out.
Finally leave the head to dry off for some hours.
Using a drier to blow air into the openings may speed up the process.
When the head is totally dry, replace it, and do head-alignment
and print a nozzle check for verification. If the check pattern is not
correct, some old and dry ink is still inside, so repeat the process.
(Old answer, see newer answer above)
From Cleaning out a Canon printhead :
Canon printers have a semi-permanent printhead. It's the 'holder' that the ink cartridges (tanks) fit into. It can be removed fairly easily. Over time, it can and does become clogged, mainly from letting the printer set without use. You need to print both black and color at least once per week in order to prevent clogging and keep the ink flowing.
Normally you can clear up minor clogs and streaking by running the cleaning cycle found in your printer's software utility. For quick access to your printer's utility (drivers): In your browser's address bar type: control panel\printers and hit 'enter' on your keyboard. This will bring up a page with your installed printer's icon. Place your mouse cursor over the printer icon and 'right-click' on it. When the dropdown menu shows, click on 'properties'. This will bring up the printer utility. Look for the 'maintenance' tab and follow the cleaning prompts.
If the cleaning cycles don't do the trick in clearing up poor and incomplete output, don't give up and assume you'll need to buy a new print head (they often cost more than a new printer!). You can try the following as it will cause no harm: (we and many others have successfully done this)
Remove the ink tanks from the printhead (tank holder) and seal them in a ziplock plastic bag. Set them aside, preferably propped upright. Next remove the print head, it snaps out. Use the kitchen or other sink and turn the water on so that it is running slowly and that it is warm. Hold the print head, bottom facing down, and cup your hand under the running water so that the water fills it up and flows into the the print head. This will flush the ink out of the printhead. You'll need to turn the print head over, bottom side up, and flush it that way as well. You'll see when there is no more ink being flushed out.
You WILL need to do a final flush/rinse with some distilled, or purified water. This ensures that any residual minerals or impurities of the tap water are flushed out of the print head. Set the print head on a paper towel to dry, drying time will vary but is LONG. TIP: To expedite the drying process, shake the printhead vigorously as there is a fair amount of water retained after rinsing!
Place the printhead (tank holder) back in the printer and reinsert the inks tanks. The printer should automatically do a print head alignment. If it doesn't, manually perform a "Printhead Alignment" through the printer's utility. Next, run the "printhead cleaning" cycle and print a nozzle check.
Here's another, similar method:
Open the lid of the printer and when the print head moves to the left, remove the cartridges (tanks). You should then be able to easily remove the print head. It has the circuitry on it and on the bottom are the outlet nozzles.
Get a small, shallow pan and lay a couple of paper towels in the bottom. Heat up (via microwave) enough purified/bottled water to cover the paper towels. Set the print head on the towels. You should see ink 'bleeding' out. Move the print head to several different spots (about a minute each spot). This will dissolve the ink clogs. Remove the print head and gently dab it dry on fresh paper towels.
Place the print head back in the printer and insert the ink tanks.Run a print head alignment and a regular cleaning followed by nozzle check and test print(s). That should do it. If not, repeat the above procedure(s).
If the above two methods don't quite do the trick, try this:
Remove the ink tanks and printhead (cartridge holder) from the printer as instructed above. Get a small bowl, dish or pan, large enough to accommodate the printhead. Lay down a layer or two of paper towels in the bottom of the container to protect the printhead circuitry from contact damage.
Make up a 50/50 mixture of ammonia and distilled water, about a pint will do. Heat the mixture up in the microwave for about a minute or so, not boiling. Slowly pour over the paper towels so that you have approximately 1/2 inch mixture total. Set the printhead (cartridge holder) into the container and pour some of the remaining mixture over the top, just enough to wet it good. Let it soak for several hours (3-4) or overnight. You can move it slightly to a different spot every hour or so.
Remove the printhead and rinse thoroughly with distilled water. Shake it dry, set it on folded paper towels to air dry. Reinsert the printhead and ink tanks. The printer should do an automatic "Printhead Alignment", if not manually do so and run a cleaning cycle or two followed by printing a nozzle check pattern to check your progress. Repeat as necessary.