This error is often caused by the optical sensors not functioning correctly. 

The 3 main causes are: 

  • A build-up of dust (which generally occurs over time) in front of the sensor which obscures the light admitted through to the sensor
  • One of the connections to the rotation position sensor is loose/disconnected
  • A fault with the sensor itself

This FAQ article shows the steps that you can take to investigate the cause of your error.