Fan speed control

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Fancontrol, part of lm_sensors, can be used to control the speed and sound of CPU/case fans. This article covers configuration/setup of the utility.

For some Dell laptops, an alternative is #i8kutils.

Sensor driver

Support for newer motherboards may not yet be in the Linux kernel. Check the official lm-sensors devices table to see if experimental drivers are available for such motherboards.

It is recommended not to use lm_sensors.service to load the needed modules for fancontrol. Instead, manually place them in /etc/modules-load.d/load_these.conf since the order in which these modules are loaded dictate the order in which the needed symlinks for hwmon get created. In other words, using the lm_sensors.service causes inconsistencies boot-to-boot which will render the configuration file for fan control worthless for a consistency point of view. To avoid this problem:

In /etc/conf.d/lm_sensors you find the modules. If not there, run as root sensors-detect accepting the defaults. In the modules-load.d file place one module name per line. Specifying them like this will create a reproducible order. Another alternative is to use absolute device names in the configuration file.[1]


Set up lm_sensors.

$ sensors
Adapter: ISA adapter
Core 0:      +29.0°C  (high = +76.0°C, crit = +100.0°C)  


Adapter: ISA adapter
Vcc:         +1.14 V  (min =  +0.00 V, max =  +4.08 V)   
VTT:         +2.08 V  (min =  +0.00 V, max =  +4.08 V)   
+3.3V:       +3.33 V  (min =  +0.00 V, max =  +4.08 V)   
NB Vcore:    +0.03 V  (min =  +0.00 V, max =  +4.08 V)   
VDRAM:       +2.13 V  (min =  +0.00 V, max =  +4.08 V)   
fan1:        690 RPM  (min =   10 RPM)
temp1:       +37.5°C  (low  = +129.5°C, high = +129.5°C)  sensor = thermistor
temp2:       +25.0°C  (low  = +127.0°C, high = +127.0°C)  sensor = thermal diode

If the output does not display an RPM value for the CPU fan, one may need to increase the fan divisor. If fan speed is shown and higher than 0, skip the next step.

Increasing fan_div

The first line of the sensors output is the chipset used by the motherboard for readings of temperatures and voltages.

Create a file in /etc/sensors.d/:

chip "coretemp-isa-*"
set fanX_div 4

Replacing coretemp-isa- with name of the chipset and X with the number of the CPU fan to change.

Save the file, and run as root:

# sensors -s

which will reload the configuration files.

Run sensors again, and check if there is an RPM readout. If not, increase the divisor to 8, 16, or 32. YMMV!


Note: Advanced users may want to skip this section and write /etc/fancontrol on their own, which also saves them from hearing all of the fans at full speed.

Once sensors are properly configured, use pwmconfig to test and configure fan speed control. The default configuration options should create /etc/fancontrol configuration file:

# pwmconfig


Warning: Some of the steps outlined below describe how to tweak fan speeds. Before doing this be sure to have a low CPU load.
Note: On several systems, the included script may report errors as it tries to calibrate fans to the respective pulse-width modulation (PWM). Users may safely ignore these errors. The problem is that the script does not wait long enough before ramping up or down the PWM.

Users wishing more more control may need to tweak the generated configuration. Here is a sample configuration file:

DEVPATH=hwmon0=devices/platform/coretemp.0 hwmon2=devices/platform/w83627ehf.656
DEVNAME=hwmon0=coretemp hwmon2=w83627dhg
FCFANS= hwmon0/device/pwm1=hwmon0/device/fan1_input
  • INTERVAL: how often the daemon should poll CPU temps and adjust fan speeds. INTERVAL is in seconds.

The rest of the configuration file is split into (at least) two values per configuration option. Each configuration option first points to a PWM device which is written to which sets the fan speed. The second "field" is the actual value to set. This allows monitoring and controlling multiple fans and temperatures.

  • FCTEMPS: The temperature input device to read for CPU temperature. The above example corresponds to /sys/class/hwmon/hwmon0/device/temp1_input.
  • FCFANS: The current fan speed, which can be read (like the temperature) in /sys/class/hwmon/hwmon0/device/fan1_input
  • MINTEMP: The temperature (°C) at which to SHUT OFF the CPU fan. Efficient CPUs often will not need a fan while idling. Be sure to set this to a temperature that you know is safe. Setting this to 0 is not recommended and may ruin your hardware!
  • MAXTEMP: The temperature (°C) at which to spin the fan at its MAXIMUM speed. This should be probably be set to perhaps 10 or 20 degrees (°C) below your CPU's critical/shutdown temperature. Setting it closer to MINTEMP will result in higher fan speeds overall.
  • MINSTOP: The PWM value at which your fan stops spinning. Each fan is a little different. Power tweakers can echo different values (between 0 and 255) to /sys/class/hwmon/hwmon0/device/pwm1 and then watch the CPU fan. When the CPU fan stops, use this value.
  • MINSTART: The PWM value at which your fan starts to spin again. This is often a higher value than MINSTOP as more voltage is required to overcome inertia.

There are also two settings fancontrol needs to verify the configuration file is still up to date. The lines start with the setting name and an equality sign, followed by groups of hwmon-class-device=setting, separated by spaces. You need to specify each setting for each hwmon class device you use anywhere in the config, or fancontrol will not work.

  • DEVPATH: Sets the physical device. You can determine this by executing the command
readlink -f /sys/class/hwmon/[your-hwmon-device]/device | sed -e 's/^\/sys\///'
  • DEVNAME: Sets the name of the device. Try:
$ sed -e 's/[[:space:]=]/_/g' /sys/class/hwmon/[your-hwmon-device]/device/name
Tip: Use MAXPWM and MINPWM options that limit fan speed range. See fancontrol manual page for details.
Tip: Not only the DEVPATH may change on reboot due to different timing of module loading, but also e.g. the temperature sensor paths (hwmon0/device/temp1_input becomes hwmon0/temp1_input). This usually happens on a kernel update. Check the system log to find out which is the troublemaker:
# systemctl status fancontrol.service
and correct your config file accordingly.


Try to run fancontrol:

# /usr/bin/fancontrol

A properly configured setup will not error out and will take control of system fans. Users should hear system fans slowing shortly after executing this command.

To make fancontrol start automatically on every boot, enable fancontrol.service.

Note: Upon upgrading/changing the kernel, running fancontrol may result in an error regarding changed device paths. This issue may be fixed by running sensors-detect and restarting the system.


i8kutilsAUR provides an alternative method of controlling the fan speed on some Dell Inspiron and Latitude laptops. It makes use of the /proc/i8k interface provided by the dell_smm_hwmon driver (formerly i8k). Results will vary depending on the exact model of laptop.


tcl must be installed in order to run i8kmon as a background service (using the --daemon option). To run the X11 desktop applet, tk is required as well.


By default, i8kmon only monitors the CPU temperature and fan speed passively. To enable its fan speed control, either run it with the --auto option or enable the option permanently in /etc/i8kutils/i8kmon.conf:

set config(auto)       1

The temperature points at which the fan changes speed can be adjusted in the same configuration file. Only three fans speeds are supported (high, low, and off). Look for a section similar to the following:

set config(0)  {{0 0}  -1  55  -1  55}
set config(1)  {{1 1}  45  75  45  75}
set config(2)  {{2 2}  65 128  65 128}

This example starts the fan at low speed when the CPU temperature reaches 55 °C, switching to high speed at 75 °C. The fan will switch back to low speed once the temperature drops to 65 °C, and turns off completely at 45 °C.

Disable BIOS fan speed control

It may be necessary to turn off control of the fan speed by the BIOS to prevent it from "fighting" with i8kmon. On some laptops, this can be done using the smm utility. This utility is extremely dangerous as it writes directly to an I/O port to invoke the processor's System Management Mode. Use it at your own risk.

smm must be compiled and installed manually. On a 64-bit system, gcc-multilib is required. Locate the file smm.c in the i8kutils source and compile it:

$ gcc -m32 -o smm smm.c

To disable BIOS fan speed control, run (as root):

# ./smm 30a3

To enable it again:

# ./smm 31a3
Note: This method may disable other power management features of the BIOS as well, such as notifying Linux when the power button is pressed.

Installation as a service

i8kmon can be started automatically as a systemd service using a unit file similar to the following:


#ExecStartPre=/usr/bin/smm 30a3  # uncomment to disable BIOS fan control
#ExecStopPost=/usr/bin/smm 31a3  # ... and re-enable it afterwards
ExecStart=/usr/bin/i8kmon -d



There are no working fan sensors, all readings are 0

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: This was done on ASUS_N550JV, which has single fan, while desktop computers can have multiple fans. (Discuss in Talk:Fan speed control#)
Warning: This section is for power users. Do not follow it if you do not know what you are doing.

First, find a directory containing your required files:

# find /sys/class/hwmon/hwmon*/ -name "fan*"

And the output should look like this:


This means that files you will need are located in /sys/class/hwmon/hwmon2/.

Now check the current fan speed:

# cat /sys/class/hwmon/hwmon2/pwm1

The output should be from 0 to 255.

Now check if you are able to control fan speed by running some of these commands (note value between 0 to 255. 0 means fan is completely stopped):

# echo "100" > /sys/class/hwmon/hwmon2/pwm1
# echo "200" > /sys/class/hwmon/hwmon2/pwm1
# echo "255" > /sys/class/hwmon/hwmon2/pwm1

If you can hear how fan noise changes according to fan speed values given above, then you are on the right path and pwmconfig indeed shows incorrect message.

Open first console and execute:

# watch -n 1 "echo 2 > /sys/class/hwmon/hwmon2/pwm1_enable"

Then open second console and execute this:

# pwmconfig

Once you are done and the configuration file is generated, you should stop the first console.