Diagnosis
The unit is reading the room's ambient humidity and won't run because it's already at your target. 'HI' means current humidity is above your set point, 'LO' means it's below.
How to Fix It
This is normal operation, not an error. If you want it to run to get humidity lower, press the ‘Humidity’ button to set a target lower than the current reading. If the display shows ‘LO’ and the room feels damp, the built-in hygrometer might be off.
If That Didn't Work
If it stubbornly shows ‘LO’ in a clearly damp room, unplug the unit for 5 minutes to reset the control board. When you plug it back in, it will recalibrate its sensor on startup.
Heads Up
The humidity sensor is inside the unit, so placement matters. Don’t put it right next to a laundry machine or shower where it gets a localized blast of moisture, or in a dead-air corner. Central location in the space gives the most accurate reading.