Hello Jerry Potter, Welcome to TSF
Your Canon« Pixma iP1500 might have reached the end of its service life.
All Pixma Printers start to give 'false errors'; when in fact, it is the absorption pad that is saturated, thus requiring replacement.
The replacement can be more uneconomical than an actual replacement printer.
I speak from experience, having had a Pixma iP4200 and a Pixma iP4300, that both died after 3 years service. (they got a thrashing in that time too).
I found out about the absorption pad from the friendly people at Canon«.
Getting to the pad is major surgery, you practically have to disassemble the entire machine.
Absorption pads for the iP4200 and iP4300 were $95ea (AUD), whereas a brand new iP4950 was only $110 (in Australia in June 2012).
Without making this an advert for Canon«, the iP4950 has disc printing capabilities as well as printing top notch photos.
All printer manufacturers today make these things to be 'throw-a-way', bad for the environment, but good for their business.
You can try accessing the absorption pad and clean it in white spirits, leaving it to dry for 24 hours, but this "fix" only lasts for about 3 months; so if money is scarse, give it a go.
Furthermore, if you are on about the sixth set of cartridges; then, its had a fair amount of service life.
Canon« says that a set of cartridges (Black and Colour) will about 470 - 590 pages (on average), so yours has had a good run.
I stress that these are my candid thoughts, and BTW I do not work for Canon« and nor have I sold their products.
Post back with your thoughts/ideas.