It usually happens with the printers that the print head if the printers get overheat and may undergo self-destruction. This issue occurs when there is no flow of ink to the print head to keep it cool. So, it probably occurs when the cartridge is completely dry or no ink is flowing through the cartridge. So when you decide to insert in it a new cartridge, it happens that the printer does not find any ink present in the printhead, and that is the time when it reports the Canon b504 error .

So, this is the root cause behind the Canon Printer Error b504. Now, here are some ways mentioned by which you can fix this error in your printer.