When should I consider the print head to be the definitive cause of B204 and what are the options?

If all other troubleshooting steps (power cycling, cleaning, checking cartridges, driver updates) fail to resolve the B204 error, it strongly suggests a problem with the print head itself. Depending on the printer model, you might be able to purchase and replace the print head yourself, or you may need to contact Canon support or a qualified repair technician for repair or replacement of the entire printer if the print head is not a user-replaceable part.