DIAGNOSIS The optical particle sensor is dirty or obstructed. It's located on the front panel near the control buttons.
FIX Unplug the unit. Use a can of compressed air to blow into the small slit of the sensor chamber. Do not use liquids or insert anything. Gently vacuum around the slit with a soft brush attachment. Plug it back in and test with a known source like dust from a blanket.
IF THAT DIDN'T WORK The sensor itself may have failed. Auto and Plasma Wave functions rely on it. You can run the unit manually, but sensor replacement means a service call.
HEADS UP Avoid placing the unit directly next to air fresheners, candles, or humidifiers. Oils and moisture can coat and ruin the optical sensor.