My Blue air purifier has started showing a red light and I am not sure what is triggering it because the air in my room does not feel dusty or heavy. I cleaned the outer pre filter and vacuumed the inside section but the red light still appears after a few minutes of use.
I noticed that the fan speed changes on its own more often now and sometimes the air coming out of the top feels weaker than usual. This makes me wonder if the sensor is misreading something in the room that I cannot see.
I also tried unplugging the purifier for a while and restarting it but the light returned almost immediately which makes me think it is not a simple reset issue.
Has anyone experienced a red light staying on with a Blue model even after cleaning the filter layers because I would appreciate some guidance on what else I should check 
I had this happen on my Blue unit and the problem was the sensor port on the side. It looked perfectly clean but there was a thin layer of dust built up inside the small opening. The purifier kept reading the room as polluted even though the air was clear.
I used a cotton swab and gently wiped the sensor chamber and the red light changed back to blue within a few minutes.
My Blue purifier stayed on red for hours and I found out that the carbon filter was overloaded with trapped odors. It did not smell bad but the material was holding onto old particles from cooking.
Replacing the carbon sheet fixed the problem instantly so it may help to check how old the filter layers are.