Faulty or oversensitive temperature sensor on 15-inch Macbook Pro?
I see two potential problems:


An "over sensitive" sensor as you described that is reporting the CPU temp as higher than what it's supposed to be


or....


A faulty CPU that has a temperature spike when you put it under load.  In this case, the sensor is working correctly.


The tech changing out the fan can do nothing to fix this problem - remember, the sensor is "reading" a temp change and telling the SMC to kick on the fans.  Changing the fan won't change how the sensor works.

What you need to do is diagnose whether or not the temp reading is accurate and whether the sensor is working or not.  You can start with Apple Hardware Test (AHT).  (Hold the D key while booting from a powered off state with the AC adapter connected) and then take it into Apple to have it diagnosed.