How can I troubleshoot the 'ribbon out' error on my Zebra ZM400 printer?

To troubleshoot the 'ribbon out' error, first verify the ribbon is correctly installed, that the ribbon is not broken, and that the ribbon supply roll isn't depleted. Next, check the ribbon sensor (usually a small arm or optical sensor) for any obstructions or dust. Clean the sensor if necessary and ensure the ribbon path allows the sensor to detect the ribbon presence. Finally, try replacing the ribbon to see if the error persists.