Is your Canon printer giving you the dreaded "Wrong Cartridge" error message when you try to print? This is not an Ink Cartridge issue but is caused by the actual print head itself. It is a known problem with many of the older Canon printers such as the MP780 and the MP730 models.
One of the main causes is that the electrical terminals which connect the printer to the print head are dirty or have become oxidised over a long period of use. The way to fix this is to remove all of the ink cartridges and then unclip and remove the actual print head.
Once the print head is removed you will discover a small green and gold circuit board which is the part needing cleaning. Give this a clean with some non corrosive cleanser or even cleaning alcohol. Once cleaned re install the print head, put the cartridges back in and give the printer another go. Most of the time this will fix the problem, if however the error message remains then its almost certain that the print head has died. If this is the case simply move on to a newer model Canon printer as printers of this age will usually develop other problems and replacing the print head is not economically viable.
For a better view of cleaning the print head contacts take a look at the following video.