Blocked Cathode Fan (reference 2 image).

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

Damaged wire unit (reference 1 & 5 image) of the Cathode Fan.

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 service manual chapter "Replacement of a Card Cage board".

Cathode fan (reference 1 image) end of life.

Replace the Cathode Fan. See service manual chapter "Replacement of the Cathode fan".