With Power OFF an unplugged... remove the magenta ribbon cables completely - as in take them out of the printer.
Then move the ribbon cables from the Cyan head to the Magenta head (while leaving the other end of the cable in the print carriage board as it was [in the cyan channel]).
Magenta head will fire in place of cyan during test print, and Cyan will not print at all. NOTE: the position of print will differ on the test print and this is normal.
If the issue goes away, you could have faulty Cyan head.