Diagnosis
That’s your unit telling you the condensate bucket is full and needs to be emptied. It’s a safety feature to prevent water overflow.
How to Fix It
First, slide the bucket straight out from the front of the unit. Go ahead and empty it. Before you slide it back in, check two things: 1) Make sure the bucket is clicked all the way into its rails. 2) Look at the little plastic tab on the front-right side inside the bucket compartment—that’s the float switch. Push it down gently and let it pop back up to make sure it’s not stuck. Slide the bucket back in firmly until you hear it click home. The display should clear and the unit will start up on its own.
If That Didn't Work
If the 'CO' code stays on with an empty, properly seated bucket, that float switch might be stuck in the 'up' position. Unplug the unit. Look inside the empty compartment and find that small, usually white, float switch. Gently wiggle it up and down a few times to free it. Plug the unit back in and see if the code clears.
Heads Up
Always handle the bucket with two hands and set it on a level surface when carrying it—the internal flap that keeps water from sloshing out isn’t perfect, and a tilt can cause a spill.