Temperature sensor disconnected.

Check if wire (reference 14 image) of the ambient temperature sensor (NTC) is connected with the Signal Backplane.

Defect temperature sensor.

Replace the ambient temperature sensor (reference 12 image).

Malfunction Cinema Control board.

Replace the Cinema Control board. See service manual chapter "Replacement of the Cinema Controller".

Malfunction Signal Backplane.

Replace the Signal Backplane. See service manual chapter "Card Cage".