DIAGNOSIS: The robot's sensor is likely clogged with fine dust or the filter isn't seated perfectly after cleaning. The error triggers based on airflow, not just visible cleanliness.
FIX: Remove the filter and give it a sharp, hard tap against your trash can or sink to dislodge the deepest dust you can't see. Don't just brush it. While it's out, use a dry cotton swab to gently clean the small sensor port inside the robot's dustbin compartment. Reinstall the filter, making sure it clicks firmly into place on all sides.
IF THAT DIDN'T WORK: The filter might be water-damaged or too worn. Even if it looks okay, its paper structure can break down. Run the robot without the filter for a 30-second test clean. If the error disappears, you need a genuine replacement filter—third-party ones often cause this.
HEADS UP: Never rinse the HEPA filter with water, only tap it clean. Washing it ruins the material and causes permanent airflow errors.