Can't control BOOSTA fan on Mac Pro 5.1 dual CPU

Mac Pro 5.1 has been cleaned and thermal paste reapplied to both CPUs and Northbridge heatsinks with no change. Also swapped out CUPA heatsink and fan again no change. No tools appear to be able to control BOOSTA fan, but can control all others. Ramping up all fans except BOOSTA has no impact either. Any comments/ideas most welcome - thanks - Eric