Hi, when it does this 99% of the time it is either bad head cable, or the print carriage board is failing.
The cyan head isn't getting the correct signal to tell it when to turn on or off, so it prints all the time over everything else.
First thing is unplug the printer from the mains ( take out the main power cable ), leave for 15 minutes, then carefully remove the head cable from the head and the carriage board and see if any of the pins look damaged.
If they look ok, the wipe them down with a clean dry soft cloth and re-install the cable and see if it fixes the problem.
If not, try swapping the cable from one of the other heads and see if the fault moves to the new head. If it does you know the cable is faulty. If the cyan is still doing the same thing, then it is most likely a problem with the print carriage board.
