If your HP C309g has a clogged printhead, this guide will help potentially resolve the problem and get the printer working again without another $80 printhead to fix the problem when you can fix it with materials you have at home. The steps to clear the printhead can also be used interchangeably with other semi permanent printhead printers. While the cleaning steps will easily carry over, you can't carry the disassembly steps and printhead bypass over. These are specific to the HP C309g. Your best bet for other brands of printer is to look online for the procedure on how to remove the printhead.
'''HP Photosmart Premium ePrint C310a/C309n owners: You can carry both the disassembly and cleaning instructions over, but I have not verified the initialization bypass on a ePrint ready model yet. You're welcome to try but it may not work on these models. If it doesn't, you will probably want new ink on hand to be sure.'''
It should also be said while this is a fix to the problem with a high probability of it working again, it will not fix physical damage to the head that causes communication problems, ink binding and physically damaged printheads causing the problem. If you mixed the ink formulations up and put the wrong ink in the cartridge, this will not help. Ink binding can't be fixed this way since it destroys the head. This is simply something to attempt before you send the printer to the recycling facility or eWaste drive.