Cause #1:
Plastic bags in filters have not been removed.

Solution #1:
Remove plastic bags from filters.
Cause #2:
Too much ventilation in the current room.
Current room has high amount of pollutants.

Solution #2: 
Change rooms and see if there is any difference in the API sensor.

Cause #3:
There's dust/dirt behind the hatch where the dust sensor is (see images below for reference)

Solution #3:
Open the hatch and blow on it or remove/clean the dirt on the hatch.
Use a dry and clean soft cloth, cotton pad or cotton bud.
Do not use wet, damp or harsh chemicals, or any rough brushes or sponges as these can damage the sensor.
Refer here for more information on how to clean the air quality sensor.

The light will turn blue again once it is cleaned.
If you have tried all of the above and the problem persists, please contact us at for further assistance.
ION390 dust sensor location: