Does it display 0 even under load?
If so, try AMD Overdrive, to make sure that the core temperatures don't display 0 there as well, when at load.
There was a thread very similar to this, sometime ago, and I can't remember if the thread-starter's question was solved by finally finding a program that would display the temperatures, or if he ended up having to replace his motherboard. (I do remember that we had told him that, if it was his CPU Core temperatures that were faulty, then it was his CPU, which was not the case. Ugh.)