I know that this is an old post, but I have to jump in here since I just installed a FX-4100 into a system for a customer that wanted a "budget" upgrade. After the install, I did a quick system check. BIOS on the ASUS motherboard listed the CPU temp at 38 degrees at boot. I ran a number of hardware monitor checks and all of them came back with a CPU temp of 13C to 15C which is impossible on air cooled heatsink since the room temperature was 22.5C. I rebooted the system and rechecked the BIOS CPU temp. Temp was at 41 C. I let the system restart and it still showed reading of 13C to 15C on Core Temp, Speccy, HWMonitor, and AMD Overdrive . Just to double check, I used my IR non-contact thermometer and check the base of the heatsink. I got reading of 39.5C to 42C.
Just for the hell of it. I reinstalled the AMD Phenom II X4 945 cpu I pulled out earlier. BIOS showed a CPU Temp of 36 C. After the system booted all the other HW monitor programs showed temp of 38C to 41C. I reinstalled the FX-4100 and got the same 13C to 15C at idle readings. Not sure what the problem is/was, I'm waiting for a new FX-4100 just to make sure. However, the customer says the systems is running fine, no crashed or heat alarms.
I really have to laugh when I read so many people posting that their idle temps are in the mid teens C. I guess if they are running their system in in a really really cold room (12C or colder), have an ice cold water cooler or if they running a liquid nitrogen they might get those reading. But on an air cooled heatsink, at normal room temperature, I don't think so. If you really want an accurate temp, get an IR (laser) non-contact thermometer or a digital thermometer and attached the probe to the base of your heat sink (not between the heat sink and cpu - that would not be good).