Malfunction Fan Control board or SMPS board.

Measure on the Signal Backplane the ++12V voltage on pin 1, 2 or 3 of the connector of the black wire unit which comes from the SMPS board. See image.

If the measured voltage is about 12V then replace the Fan Control board. See "Replacement of a Card Cage board". Otherwise replace the SMPS board. See "Replacement of the SMPS board".

Short circuit or bad connection.

Check the Signal Backplane for bad connections.