~ # fan_control -?
     *** Fan Cobtrol Help Message ***
     fan_control b c: for NAS booting
     fan_control 0 d : [auto: low/medium/high/max] open debug msg
     fan_control 0 c : [auto: low/medium/high/max] close debug msg
     fan_control -L [value] : set Lower (THYST)
     fan_control -H [value] : set Upper(TOS)
     fan_control -g 0 : get current temperature
     fan_control -g 1 : get Lower temperature
     fan_control -g 2 : get Upper temperature
     fan_control -g 3 : get fan state
     fan_control -g 4 : get fan rpm
     fan_control -f 0 : set fan stop
     fan_control -f 1 : set fan rpm4000
     fan_control -f 2 : set fan rpm5000
     fan_control -f 3 : set fan rpm6000
     fan_control -f 4 : set fan rpm7000
     fan_control -f 5 : set fan rpm8000
     fan_control -f 6 : set fan rpm9000

The command "fan_control -f 3" can manually turn it on. Its noise and air flow can be easily detected. The larger the number, the faster is the fan speed. The command "fan_control -f 0" can manually turn it off. The fan is working !!!

The commands "fan_control -g 0" would give the hard drive temperature. "fan_control -g 4" give the fan RPM. The figures are the same as in the dashboard.

Using these commands, I have managed to bring down the temperature from 55 back to 45 manually.

So the problem is not in the fan but in the automatic temperature control. Perhaps some DIY people can write a background monitor program to check the temperature and change the fan speed periodically.

Another finding is a file called /etc/fan_temperature.txt  Its contents is here:
     ~ # cat /etc/fan_temperature.txt
     TEMPERATURE_STOP = "39"
     TEMPERATURE_LOW = "45"
     TEMPERATURE_HIGH = "49