Try the diagnostic. You may have a disconnected temperature sensor.
Apple Hardware Test is:
1) included on the model-specific Installer DVDs that shipped with your Mac.
2) included on your ORIGINAL Hard Drive -- provided you are still using the same Hard drive that shipped with your Mac.
3) not available elsewhere. You may need to carry your Mac Pro to the Genius Bar and use theirs. The appointment is free, in warranty or out.