Yeah, this one's common. The air quality sensor lens is probably dirty, or the sensitivity needs adjusting.
First, clean the sensor. Unplug the unit. Open the air quality sensor cover on the right side. Use a soft brush or vacuum on the air inlet and outlet vents near it. Then, lightly wet a cotton swab and clean the lens, dust inlet, and dust outlet. Follow up with a dry swab. Do this every 2 months.
If it's still stuck on red after 2 hours of operation, you can adjust the sensor sensitivity. While the unit is on, press and hold the Speed button for 2 seconds. The indicator will cycle through sensitivity settings (Insensitive, Standard, Sensitive). Release the button when you reach the desired setting. Try 'Insensitive' if your room is genuinely clean.
HEADS UP: Install the purifier at least 2 meters away from TVs or other electronics, and away from strong artificial light or direct sunlight, as these can interfere with the sensor.