I recently installed Ubuntu 14.04 and got the package lm-sensors. I mainly did this to monitor if my laptop was overheating. Each time I run it, it shows a very low temperature reading (the same every time) of -128.0 degrees celcius. Why does this happen?

  • 2
    It means it's not reading the sensor. – Matt H May 19 '14 at 22:17
  • Any way to make sure that it reads the sensor? – Colorless Photon May 19 '14 at 23:12
  • I think it depends on the which motherboard chipset you have. Google the chipset and lm-sensors and it might just be that you have to load a module and add it to /etc/modules – Matt H May 19 '14 at 23:43
  • It reads the sensor but the sensor is off, see my answer... – Rmano May 20 '14 at 4:19
up vote 9 down vote accepted

I think it is supposed to be like that. New kernel 3.13 has an advanced dynamic power management of the ATI card with the radeon driver. In normal conditions the ATI discrete is off:

[:~/Pictures/2014] 1 % sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynOff:0000:01:00.0
1:IGD:+:Pwr:0000:00:02.0

so when we read the sensors:

[:~/Pictures/2014] % sensors                                        
[radeon-pci-0100
Adapter: PCI adapter
temp1:       -128.0°C  (crit = +120.0°C, hyst = +90.0°C)...]

The sensor read nothing --- the card is off, its sensor too.

If you start the card (see for example this blog entry), for example by opening another windows and starting in it:

DRI_PRIME=1 glxgears -info

and check again, you'll have:

[:~/Pictures/2014] % sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:01:00.0
1:IGD:+:Pwr:0000:00:02.0

the card is ON and...

[:~/Pictures/2014] % sensors                                        
radeon-pci-0100
Adapter: PCI adapter
temp1:        +65.0°C  (crit = +120.0°C, hyst = +90.0°C)

...the sensor too.

A lot of info is from this Arch forum entry and this Arch doc page (hat off to Arch for the quality of documentation and for having it up-to-date...).

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