I have the exact samt problem as you had (/have). It works for me in rivatuner, so there's no problem with the sensor. However, in rivatuner I cannot get the fan control options working. (my goal is to make dynamically adjusted fan speeds) So then I use nTune, where I can set the fan speed. But then nTune (or rather NVMonitor) cannot detect the gpu temp. It just says 0 degrees.

Did you just come to the conclusion that NVMonitor does not work with this specific graphics card?