Diagnosis
The filter timer has reached zero. The red ring around the control knob is a visual indicator only; it does not affect performance.
How to Fix It
With the unit powered ON, press and hold the fan speed button (the 'i' button) for 5 seconds. You will see the red filter indicator light turn off. This resets the 12-month timer. The light will not turn off if you attempt the reset while the unit is powered off.
If That Didn't Work
Ensure you are pressing the correct button. It is the smaller 'i' button to the right of the main power/knob, not the power button itself. The hold time must be a full 5 seconds.
Heads Up
Reset the light only after installing a new, genuine Blueair filter. Resetting it with an old filter will give you inaccurate maintenance timing.