Why are the temperature values for some of my FB-DIM modules in the Mac Pro or Xserve missing while others are displayed fine?

This indicates that you are using FB-DIMMs which are not fully compatible with Macintosh systems. In most cases, this is caused by errors in the firmware of the AMB chip (Advanced Memory Buffer) controlling the module. You should replace these incompatible modules: If your system is not receiving valid temperature readings from the FB-DIMMs, the cooling control for the RAM temperature zone won’t work correctly.