I have an AMD FX -8120 processor on an MSI 990FXAGD65 motherboard with 16GB of Patriot RAM. Sometimes when my PC boots, Core Temp reports a CPU temperature of 255 C, which causes the CPU to throttle to 1.4GHz. When I reboot, it returns to normal.
I've scoured the Internet, emailed with AMD and MSI, and have only tried one thing that made sense. I set the Windows power setting to High Performance, and set minimum CPU power to 100 percent, which should have set the CPU clock cycle to 3.1GHz.
The Internet forums I've found claim that because the temp is reading 255 C, the BIOS is stepping down the CPU to its lowest cycle even if Windows set that minimum higher. That doesn't make sense, as I would hope the BIOS shuts down the rig if the temperature is 2.5 times the boiling point of water. This happens intermittently, and I've gone months without it occurring. But this week it came back three times. —Marty
The Doctor responds: Your CPU isn't actually hitting 255 C, of course. 255 happens to be the highest possible value of an unsigned 8-bit integer, which is probably what the sensors use to report temperature, so it looks like a software error. This seems to happen with some regularity on MSI boards and FX-class AMD CPUs, if forum threads are any indication.
Per an AnandTech thread, it looks like the CPUs are drawing more power than the board can provide (especially at startup), so to prevent damage to the MOSFET, MSI has the sensor report a reading of 255 C so the CPU throttles down and draws less power. The weird thing is that this happens more often with the FX-8350 on a 970 board than the earlier FX-8120 on the 990FX, but eitherway, it looks like an issue MSI is aware of.
Since this seems to be an inherent flaw with the board/CPU combination, you could try RMAing until you get a combo that works, but the fact that this is intermittent is interesting. The Doc doesn't know the cooling setup in your rig; it's possible that pointing a fan directly at the MOSFETs (left of the CPU) can help keep them cool during high power draw, which hopefully would stop triggering the failsafe that's making your temp sensors read 255 C. You could also try undervolting your CPU core voltage.
