Mimaki printer using more cyan from one cartridge then the other?

Take the cartridge out and rock it back and forth. You should be able to tell about how full it is from that. If there is still a bunch of ink in it, it's probably a bad sensor or defective cartridge.

 

Yeah you have to have both Cyans in for it to work. That was just to test it out. The on;y thing I can think of that may be happening is you have an old worn out damper and it's allowing ink to fall out of the head when it is just sitting there. Otherwise, both cyan cartridges should empty at about the same rate since they are used identically.

 

i don't think the dampers just simply allow the ink the "fall out" of the printhead. the system is open/closed via the capping station & ink pump operations, refreshing the siphon-effect of the ink supply, but stops/closes the flow when parked on the capping station with "closed" inkpump lines beneath it.
my guess would be you have a bad ink pump that's either not closing, or has a hole/crack in the ink line, that allows the ink to continue siphoning out through the printhead/cappingstation. if its not in your ink waste tank, all that missing cyan might be found in the tray that sits below the ink pumps inside of your printer (accessible via a back cover panel on the cappingstation-side of the printer)