The lamp house is not correctly installed in its compartment.

Check if the lamp house is properly installed. Ensure that the three fixation screws (reference 1 image "Lamp house switch") of the lamp house are fastened.

Switch damaged (reference 2 image "Lamp house switch").

Check lamp house detection switch for damage. The switch is located at the bottom right corner. Replace if damaged.

Switch disconnected (reference 3 image "Lamp house switch").

Check connection of lamp house detection switch at the Signal Backplane, if connections are OK then measure switch cable for continuity.