My CPUs idle tempurature is in the teens and sometimes lower with the stock cooler. My buddy says that this is impossible and there is something wrong with the sensor. Are these tempuratures common?
It's impossible for the CPU to idle lower than the ambient temperature.
If your room is freezing cold, then it's entirely possible to idle in the teens, but those are pretty uncommon temps. My 2500k idles around 20-22c when my room drops to about 16c at night, but I also have 4.5GHz OC cooled by a Xigmatek Dark Knight.
Right now I'm at 3.8 running Prime95 and the max I have gotten so far is 57c. I also have a 200mm fan pumping air in and another going out, plus a 120mm going out of the case
That does seem awfully cool for stock cooling (but not impossible). Again, it heavily depends on ambient temps, as well as airflow/fan speed. I had an A8 5600k running prime, and got up well past 70c, but I only had the CPU fan running at 25%, and the airflow wasn't great, and it was 68F.
I also had a strange issue with the APU where none of the typical CPU temperature monitoring softwares gave me an accurate reading (reading 0c at idle), so I had to use GPU-z to get temp readings. I, personally, use CoreTemp for recording temps.