Why CPU fan speed always the same even after I installed a system fan?

In general, CPU fan speed is changed dynamically with CPU temprature . But basiclly there are 2 modes of CPU fan; Fixed speed or temprature controlled . Even you installed a system fan to improve the system temprature but CPU fan still controlled by system BIOS. If it's fix speed, then temprature will not change with system temprature.
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