Is there any way to ignore a heat sensor on a macbook pro?
I have the same problem.  I used this program.  You can set which sensor (only one as far as I can tell) to tip the fan to higher speeds.  Best solution I could find, but if one of your other chips overheats, you might be out of luck.

http://www.crystalidea.com/macs-fan-control?ref=fancontrol_mac