Thank you all for taking a time to reply me. Basically the printer is working fine except yellow color in last two weeks. One day the error of "supply ink: Y" showed up. After we executed the function of "Sub tank" Refill. The yellow nozzle of the test print went back to normal. But the right state only last after getting done one printing job -- yellow ink dropped off suddenly again. I guess that little of yellow ink left in subtank. We never replace subtank ourselves. Is it hard to change a new subtank?
Changing subtank is pretty easy like Smoke said
just start the printer and hold remote and function buttons untill you hear a beeping, so the printer will start in maintenance mode.
When its started and ready, press function - #adjust - Subtank - discharge. This way the subtanks will fully discharge all the ink in it, turn off the main switch and you are ready to swap the subtanks.
take off the inktubes and airtubes on the top, take of the subtank sensors on the front and back (4x) and unscrew the 2 plates on the sides (1 screw each), and press the levers on the bottom to take the subtank of.
Take a good look on the numbers that are on the inktubes and sensors, because you have to place them back on the right position.
put on the new subtank and re-connect everything again.
Turn the machine on again in maintenance mode. You are ask about the inksettings, confirm and refill.
After that do a airpurge and it should all be good again.