Lamp House is not correctly installed.

Check if the Lamp House is properly installed. Make sure that both fixation screws (reference 1 image) at the base of the Lamp House are fastened. See service manual chapter "Installation of the Lamp House".

Wire of the micro switch disconnected from the Signal Backplane.

Remove the Lamp House and the lamp Cathode Fan assembly and check if the wire of the micro switch is plugged in on the Signal Backplane (reference 2 image).

Defect micro switch.

Replace the micro switch (reference 3 image).