I swapped the lower hard drive in a mini mid 2010 server to a SSD drive. I removed the temperature sensor very carefully and do not think I damaged it. After re-assembly the temperature in lower bay is reported as 0 degrees. Temperature for upper bay (untouched HD) is reported as normal. Fan operation is normal too, ie no full speed cooling.

What could cause this?

If I failed to re-connect the sensor properly I assume fans would go on full?