The yellow ink stopped printing on my mp620, i've replaced the ink cartridge even though it was quite full and run all the diagnostics/cleaning programs. it's a tiny bit better but all my prints are coming out purple/magenta

Replace print head(black & yellow) or try to clean the nozzle.(try youtube videos for cleaning)

Inkjet printers need to be used frequently to prevent ink drying out and blocking the print head nozzles.
If the print head cleaning maintenance routine does not clear the blockages, then print head needs replacing, but it may be expensive.
It may be more cost effective to buy a later model and better printer than replace the print head.

under warranty get help from a warranty call from an authorized service agent

Try the following steps.
1) remove the ink cartride and ckeck for damages
2) remove the ink cartridge and replace it securly.
Else the ink cartridge is faulty.

you have air in the air line tubes. you can try doing a couple cleaning, wait 24 hrs, and try a couple more cleanings and then do a test print.

you can try doing a ink purge, but it will **** up a lot of ink, but it might be your last resort.

press menu, and then *2864 quickly, then type 76, use the arrows to select black and then press 0.
when all done press 99 to exit.

It may work it may not, its possible you may even need to do it again, however, your inks will be depleted.

alcohol burnout print nozzles, you have ruined your printer head. you need to replace-it.

If you have run the nozzle cleaning utility, and you are not getting any yellow output, the cartridge is most likely defective. This also occurs if you are not using OEM ink cartridges.

I run about 100+ canon printers from s800 up to the new ip4500.
I have met and solved almost every problem I find described on the web. Your problem with the disappearing yellow ink can be only one thing. The ink cartridge has not been sealed after you injected the ink. If there is even the smallest avenue for air to get into the cartridge it will allow the ink to flow very rapidly back out of the cartridge and into the waste ink pads in the bottom of the printer. You won't be able to see a puddle of ink under the printer until much later when you have done many cleanings or installed many leaky cartridges.

I had the same problem with the yellow cartridge. Yellow quit printing, put in a new yellow and still would not print yellow. I had a help file, don't know where I got it, but it suggested replacing the black ink cartridge also, even though the black wasn't low yet. Replaced the black, ran a few nozzle cleans and everything worked fine. Don't know why. Try it