Reason:

(1) The changeover switch on the system controller has been changed.

(2) Sensor or low-noise cable is faulty.

(3) Wrong sensor settings.

Solution:

(1) Check the status of the changeover switch on the system controller.
Click here for details. What to do when a loop check error occurs (1)

(2) Replace the sensor or low noise cable.
Click here for details. What to do when a loop check error occurs (2)

(3) Check the sensor setting.
Click here for details. What to do when a loop check error occurs (3)