Power supply fan

My friend just bought a built PC, and the fan on the back of the power supply doesn't turn on.  The guy who made it said the fan only activates when it gets too hot.
I haven't seen it turn on once, and I played Half Life 2 for 2 hours on it, with a CPU temperature reaching 50C.  Think I have a dead fan, or is it not getting hot enough to turn on?

50C is not all that hot for a CPU running full-pelt. Although the PSU fan will turn on based on internal case temperature rather than CPU core temperature. I wouldn't worry as it seems your computer as a whole is getting adequate cooling.

My pick is: Your friend has a dead PSU, it happens some times even with new hardware (i work as a technician for a major electronic retailer) even if it's the best quality ever or the worst (acer....)

I'd check with the technical specifications of the PSU your friend bought, if the device has a thermal sensor or not.

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