The Light Processor Unit is not correctly installed.

Check if the Light Processor Unit is properly installed. Ensure that both fixation screws at the foot of the Light Processor Unit are fastened.

Switch disconnected (reference 1 on image "Light processor switch connection") on Signal Backplane.

Check connection of engine switch at the signal backplane, if connections are OK, measure switch at connector for continuity.

Damaged switch.

Remove the Light Processor Unit and check that switch is not damaged. The switch is located behind left hand side set pin which guides the engine into place. Replace switch if needed.

Malfunction Fan Control board.

Replace the Fan Control board. See "Removing a board in the card cage".