C19 refers to an excessive drying load on Hitachi units that document it. Wash capacity does not determine how much can be dried in the same program.
The Hitachi chart links C19 to the amount of laundry for drying. Confirm that your model includes this function and this alert.
Adjust the load to the selected drying setting
Check the program-specific capacity in the manual. Do not use a fixed percentage of the wash capacity: it can vary with the function and fabrics.
Pause as instructed by the unit and wait for the drum to stop, the laundry to cool, and the door to unlock. Remove part of the load when it is safe and spread the remaining items out without packing them tightly. Do not extend the heat in an attempt to dry an amount the appliance has just rejected.
If C19 appears again with a load within the appropriate capacity, record the approximate amount, type of laundry, and program for support. The message does not by itself diagnose a faulty element or let you shift the check to a standalone dryer.
The next test should respect the program’s drying limit, not just the kilograms printed for washing.