HP Designjet 510 Printer series - Manual (Page 86)

Unknown printhead error
An unknown printhead error occurs when the one of the printheads is causing a
problem but the system cannot detect which one it is. Whenever this happens the
front panel displays this error message. The number of printheads with unknown
errors will either be all the color printheads or all the printheads.
Use the following procedure to isolate the faulty printhead.
1. Select Replace Printheads from the front panel.
2. Open the carriage cover and remove all the printheads.
3. Reinsert the Cyan printhead only and close the carriage cover and the window.
4. Check the front panel—if the status of the Cyan printhead is X or ?, the conclusion is that the Cyan printhead
is faulty and must be replaced. If the status of the Cyan printhead is OK, the conclusion is that the Cyan
printhead is working correctly. Repeat the process from step 1 for each of the other printheads to identify
which one is failing.