Blocked Cathode Fan (reference 2 image "Cathode fan connections").

Check if the Cathode Fan is not blocked. Ensure that the Cathode Fan can turn freely.

Damaged wire unit.

Check if the wire unit of the Cathode Fan is not damaged. Repair if possible, otherwise replace with new one.

Malfunction Fan Control board.

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

Cathode fan (reference 1 image "Cathode fan connections") end of life.

Replace the Cathode Fan. See "Lamp cathode fan".