Blocked fan (reference 1image "Heat exchanger fan connection").

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

Damaged wire unit.

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 "Removing a board in the card cage".

Fan end of life.

Replace the fan. See "Heat exchanger fan".