Diagnosis
The robot's infrared cliff sensors are dirty or obstructed. Wiping the clear plastic covers is not sufficient if the sensor lens underneath is obscured.
How to Fix It
Power off and flip the robot. Locate the four clear plastic sensor windows on the underside. Use a dry cotton swab to gently clean the actual sensor lens inside each window. Ensure no stray brush bristles are dangling over the sensors. Power on.
If That Didn't Work
The error can be triggered by a very dark or solid black floor surface that absorbs the infrared light. Place a light-colored mat or run the robot in a different room to test.
Heads Up
Do not use liquids or cleaners on the sensor windows. Moisture can damage the internal electronics.