How do I clean out the waste tank? (Canon mp390)?

The waste 'tank' should be located on the extreme right, where the printing head comes to rest.
When the print head comes to rest, it's LOCKED in that position.
You will have to unlock the printhead and slide it to the side so you can access the 'tank'.

In some models it's easily accessible when you remove the top cover(s). Either 4 or 6 Philips screws hold it.

NB. I don't believe this is a user serviceable operation.
Could also be the actual sensor needs cleaning.

You may want to ask them as well if there is a replacement 'tank' available that you can purchase.
This will give you an idea if this in fact can be serviced by a user. :)

If it still works to your satisfaction, I'd just ignore the message until it stops working.

faxmando@yahoo.com
December 2008