1. Check the type of media that is being used since the Line Sensor may have problems detecting transparent media or some types of Non-HP media. Try loading white HP media in to the Printer and check if the Line Sensor detects it.

2. Excessive ink deposits on the Print Platen surface can fool the sensor by reflecting the light. Clean the Print Platen.

3. The Line Sensor is damaged or faulty. Replace the Carriage Assembly » Carriage Assembly (Including Belt).