If none of the tricks work then you may need to replace the entire sensor module. They can weaken over time. I replaced one in my older unit and the air readings became accurate again. It is not expensive if you find the part online 
I will try cleaning the sensor first
Sometimes the purifier reads red during carpet cleaning. Vacuum cleaners push fine dust into the air even if the room looks clean. If you vacuumed recently then let the purifier run for a while to see if the reading changes.
Finally check that the back panel clicks in place firmly. A loose panel lets air leak and the sensor loses accuracy. Once I pressed the back panel fully in the red light turned yellow within seconds.
I will check the back panel too
I also want to mention that some cooking oils create particles that rise and spread into nearby rooms. Even light frying can make the red light stay on. If your kitchen is close to your room then this might be the cause.