DIAGNOSIS The laser particle sensor at the front is dirty. It is detecting false particles (dust on the lens) and reacting to them.
FIX Power off and unplug the machine. Use a dry, soft microfiber cloth to gently wipe the small, dark sensor lens located inside the front grille. Do not use liquids or compressed air directly on it.
IF THAT DIDN'T WORK The sensor itself may be faulty. You can test by covering the sensor with your hand while in Auto mode. If the fan speed does not change, the sensor is not working. This requires a professional repair.
HEADS UP Avoid using aerosol sprays, incense, or smoking near the purifier. These can coat the sensor with a film that is hard to clean off, causing permanent calibration issues.