What Does Each Fan Cool?

The fans cool the GPU, CPU, power supply, HD and logic board. The sensors are placed to monitor these areas along with the LED/LCD Optical Drive.… if a sensor fails the fans, by default, go into high speed mode.

Fans have finite lifetimes in run hours. running them when not required, or all the time will shorten that life. Manual control of fans IMnsHO is not a good idea or practice. you might have both fans fail at the same time, resulting in the cooking of vital components.

You could already have a failing fan or sensor (even new parts fail that's why there's a warranty)… run AHT 2 0r 3 times in succession (often with heat problems running it once does not stress the system enough to find failing components). A machine this age may still have warranty and a trip to an Apple store or Certified service center would be my first step… not the drastic action you are contemplating.

If this answer is acceptable please remember to return and mark it.