Is my PSU bad? System seems to get power OK but PSU tester fails the PG test... but all voltage rails measure ok.

Jan 3, 2009
I have been having a mountain of issues with my system. It started when I tried to replace a drive and it would no longer turn on.

After stripping it down to essentials, it would turn on but would display no video, so I plugged my GPU back in and I would get video.

However, after this I was getting all sorts of random problems. The motherboard would not post about 10-20% of the time, I would sometimes get all sorts of odd POST error messages that made no sense (such as claiming the CPU had been changed when it had not been touched in years, or case intrusion detection went off when neither the motherboard nor case has that feature).

It also almost never manages to post if I have any SATA drives connected, even if it's just an optical drive.

I can get it to boot from USB however, so I used that first to run memtest86+, no problems after 12+ hours and three cycles. Then I booted the usb drive into Linux and ran 36 hours of Prime95, no issues, and then added two hours of Furmark on top of that, no issues. So I figured if memtest86+ showed no errors, and the stress of Ptime95 along with furmark gave no errors, this means the CPU, GPU, RAM, and PSU must be ok and likely the motherboard has gone bad, especially since it has been on for almost a week now running off that Linux USB. Once it POSTs it seems to run fine, but it's getting it to complete POST in the first place that is the problem.

As I was taking it apart and disconnecting everything to get it ready to rebuild with a new motherboard however, since everything was now disconnected I figured it wouldn't hurt to plug it into a new PSU tester I got during this mess, and to my surprise it fails:

The thing is, all the voltage rails appear to be fine according to that readout. Yet, the "Power Good" signal is what is failing, which doesn't really make sense if all the voltages check out. I am confused now if the PSU could be the culprit, the motherboard, or both. The PSU is also still under warranty, would this be enough to count for a warranty replacement?
Power supplies can have faults that aren't bad enough to prevent them from powering up and even put out what appear to be good voltages. The internal protection circuitry has detected some sort of fault, which is preventing the power good signal from going to 5v, or near that.

I wouldn't bother to try diagnosing it if it's still under warranty, just send it back.
Turns out it was not under warranty anymore. I thought it had a 10 year warranty, turns out it was 7 year.... and the PSU is 8 years old.