A few weeks the black on my printer stopped coming through and no matter how much cleaning/deep cleaning I did it failed to come through.
Canons official help was:
1. Use a genuine cartridge (of course I was - not)
2. Un-install and re-install the driver.
3. Download the latest driver and repeat 2.
At this point one feels like giving the tech guy a slap to wake him up to the fact that logically none of the above can be the cause as it was working fine for 3 years before!
So a bit of research later (all of 5 minutes) and I saved myself £59 plus the cost of parts (£51) plus tax and was without the use of the printer for 4 hours.
Step 1. Remove the ink tanks and seal the outlets with sellotape.
Step 2. Take out the printhead from the printer (lift the lever to unclamp it).
Step 3. Fill a bowl with warm water, hold the printhead so that it is in a horizontal position (as opposed to vertical when in the printer) and move it back and forth under the water. What you are looking for is to push water through the inlets (where the outlets of the ink cartridges sit) and out the jets underneath on one stroke and then reversed on the next stroke.
The water will get very black from the ink getting forced out and you may need to refill the bowl with clean water a couple of times.
Step 4. Run the tap with a gentle warm water flow - NOT on full unless you want water and ink splashing everywhere and ink can stain! Now holding the printhead under the water stream so that you are forcing a flow of clean water through the inlets and then flip it over so that you are forcing water the reverse way.(IMG:http://img148.imageshack.us/img148/6123/printheadclean4lh6.jpg
I also had a can of compressed air which I used to help force a bit of water through but I think I could have managed without it.
Eventually no more ink will come out either way meaning that you have completely purged the heads.
Step 5. Shake most of the water out, leave to dry for a short while and shake again to get water out from everywhere it might be - I also used a hairdryer to speed the process.
Once you are sure it is completely dry on the outside re-install it and then put your ink carts back in, it took me 3 cleaning cycles before ink came out properly (some water will be inside and has to be forced out by the ink) and voila! I have a fully functioning printer again.
If ink does not flow please check that you DID REMOVE the sellotape you first sealed the carts with
This was done on a Canon i850 which is basically the same as all the i and ip pixma machines AND should work for at least all Canon and Epson printers that have removeable printheads - NOT good for cartridges that have the printheads attached.
JUST BE VERY CAREFUL OF KNOCKING THE CIRCUITRY AND CHIPS ON THE PRINTHEAD - THESE CAN DAMAGE FAIRLY EASILY, WATER IS OK.