the thermal protect/heat sensors are on various components throughout the motherboard - that's common sense.
They are usually internal and built-in the CELL/RSX/EE since an external sensor would not be anywhere near as accurate. Those give the fan speed commands to the power controller and thus activates the proper fan RPM.
unless it's the gray (or white, I forgot sorry) fan wire which shorted or something, variable voltages passes through that to control the speed of the motor itself - you're just in need of another fan.
However, to be on the safe side and not void your warranty, just buy one of those attachable cooling units.