DIAGNOSIS The air quality indicator is locked on red. The particle sensor is either obstructed or malfunctioning.
FIX Power cycle the unit. Unplug it for 60 seconds. Plug it back in. Let it run a full cycle. This resets the sensor.
IF THAT DIDN'T WORK Check the sensor inlet on the front panel. Use a can of compressed air to blow dust out of the small sensor opening. Do not insert anything into the hole.
HEADS UP A permanently red light does not affect filtration performance. The fan will still run on your selected speed.