HP Color LaserJet 8550 Multifunction Printer series - Manual (Page 293)

EN
Service-call message 293
Service-call message
If the printer unit and the copy module are not connected properly, or
a malfunction occurs and the copy module will not operate normally, a
service message, like the one below, appears.
Figure 63 Service-call message on the touch panel display
Note The most common service-call message is E240, which means the
copy module and the printer unit are not connected. Check that the
relay cord is fastened securely in both the copy module and the
printer unit.