HP Designjet 4000 Printer series - Manual (Page 119)

8. Take special care to clean all contacts thoroughly, including the ones at the
lowest point of the connector.
9. Using the same sponge, clean the lower strip of electrical contacts on the
printhead (unless the printhead is new), avoid touching the upper set of electrical
contacts.
Caution Do not touch the surface of the printhead containing the
nozzles, which are easily damaged.
10. After waiting a few moments to allow both connectors to dry, replace the
printhead into the printhead carriage. See “How do I insert a
printhead?” on page 110.
117
How do
I
handle
the
ink
system?