The CO2 meter should be empty most of the time, indicating a full supply of breathable air. Surfacing should take care of this. The longer you run at depth, the more carbon dioxide will build up, leading to bad things

. It does seem counterintuitive for 3 of your 4 meters to read near full almost all the time with one that should read near empty most of the time. Don't worry, if the CO2 meter is empty, you're fine. Just make sure you start heading toward the surface when it starts reading more than about 80%.