I have a Dell Latitude D630 and suffers overheating problems involving the Aux temp.
First of all the heatsinks and fans are cleaned. I used CPUID Hardware Monitor to check my temps, and this Aux temperature is the only thing that keeps getting higher and higher from 40C to 90C degrees when freshly booted.
CPUID reads these temperatures for my motherboard:
When the Aux reaches 65C to 70C it underclocks the processor to 800mhz and stays there which makes the OS to respond very slowly (Which can be solved by RMClock). When it climbs to 89C-90C, after a minute or so fans start to noise up, then after a couple of seconds, the laptop shuts down; No BSOD or error messages, battery or AC mode.