Blocked fan (reference 4 image).

Unblock the fan. Ensure that the fan can turn freely.

Damaged wire unit (reference 3 & 5image)

Check if the wire unit of the 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".

Fan end of life.

Replace the fan. See service manual chapter "Replacement of the SPG/Cold Mirror fan".