My levoit air purifier turned on the red light and it has stayed that way for two days. I thought it would reset after running for a while but nothing changed. The air in my room feels heavier than normal and the purifier does not push out as much air as it used to. I removed the outer filter cover and tapped it gently to see if extra dust would fall off. It looked dirty but not terrible so I am confused about why the light is still red. I am starting to wonder if the sensor inside the machine is stuck. I read somewhere that dust can block the sensor window and cause wrong readings. I feel unsure about opening the unit because I do not want to damage anything. I wish I knew if the red light means danger or just maintenance. If anyone has advice on how to fix this without breaking the purifier then I would appreciate the help 
I had the same issue and the problem was the fine dust stuck around the sensor window. I opened the side panel carefully and used a soft brush to clean the small square opening where the sensor looks out. After I did that the red light changed to yellow and then blue after a few minutes. It might work for you too.
Try replacing the filter even if it does not look too dirty. I learned that filters can clog deep inside and it is not always visible. My red light stayed on until I installed a fresh filter and then the airflow became stronger again. It is worth trying if your filter has been in use for many months.
Your room air quality might actually be poor. I noticed that my purifier stayed red during cooking times because the air was full of tiny particles. When the room cleared up the light went back to blue. You can try running the purifier in another room to see if the color changes. That will tell you if the issue is the sensor or the environment.
I will test it in another room
If you want a simple reset trick then unplug the purifier for thirty seconds. Hold the power button for ten seconds while it is unplugged to drain leftover charge. Then plug it back in. This fixed my stuck red light once when the sensor froze.
I opened my unit because the red light stayed on and I found a clump of dust inside the corner near the fan. It was blocking air movement. After removing that dust the purifier worked normally ars take pictures so you know how to put things back.
Be sure the filter is installed in the correct direction. I made the mistake of flipping my filter and it caused strange readings including a red light. The airflow was also weak because the purifier could not pull air properly. After fixing the orientation everything returned to normal 
You might want to check the rubber seal around the filter. If the seal is not tight then outside dust sneaks in and confuses the sensor. I pressed the seal firmly into place and my red light changed after a few minutes.
I once cleaned the sensor with a cotton swab but I did not realize I pushed dust deeper inside. The reading became worse and the red light stayed on all day. A soft brush works better because it removes dust without pressure. Try brushing gently instead of poking inside.
One reason for constant red light is scented candles. They release tiny particles that stay in the air for hours. My purifier thought the air was dirty long after the candle was out. If you burned candles or incense recently then that might be why the light will not change.
If your purifier is older than three years then the motor might be getting weak. When the motor slows down the sensor thinks the air is not moving and shows a red alert. I replaced the motor capacitor in mine and the light behavior improved.
I did not know the motor could change the sensor reading
Do you hear more noise than usual. My purifier had a loud sound and the red light stayed on because hair was wrapped around the fan blade. After removing the hair the air flow returned. A simple flashlight check can reveal if anything is stuck there.
The activated carbon layer might be saturated. When that layer is full it releases a dull smell that triggers the sensor. I removed the carbon part and the smell faded after a few minutes. A new carbon layer fixed the red light issue.
Try using a vacuum on the outside vents. Some air purifiers pull dust into the vent edges and the sensor struggles because of restricted airflow. A quick vacuum helped my unit return to normal operation.
Sensor drift can happen over time. The purifier might think the air is dirtier than it is. You can try placing it near an open window for ten minutes. If the light stays red even with fresh air then it is likely a sensor blockage or filter issue.
I had success by checking the bottom compartment. Dust collects there in layers and it affects airflow. When I brushed the bottom area thoroughly the purifier responded much faster to air changes.
There is a chance that moisture affected the inside of the purifier. High humidity can cause condensation that clouds the sensor lens. Point a small fan at the purifier for a while to dry it. This fixed my stuck red light once after a humid night.
My last purifier stayed red because of a small crack in the filter frame. Air leaked around it and dust bypassed the filter. Make sure your filter frame is not damaged. Even a tiny crack can confuse the sensor for hours.