Configuration:
CPU: Intel Core i7-4790K
Motherboard: ASUS Z97-A ATX DDR3 2600 LGA 1150
RAM: Crucial Ballistix Sport XT 16GB Kit (8GBx2) DDR3 1866
PSU: Corsair RM750
When I turn the PC on, the CPU fan turns on for about one second. Also, the EZ XMP LED(XLED1) turns green for a second.
This happens about every 3 seconds until I turn off the PSU.
I disconnected all the drives and GPU, but that didn't change anything.
The only time when the CPU fan turns continuously is when I unplug the 8-pin CPU connector from PSU, so that only the the 24-pin MOBO connector is plugged in.
Trying to fix this, I got a new PSU (same model), but that didn't help.
Sorry if I missed any details. I will be happy to provide them.
Any suggestions on how to troubleshoot this?
Update: Sorry, I forgot to mention in the original post that I had cleared CMOS a couple of times. That didn't help.
Update 2: Again, sorry I didn't explicitly mention in the original post that all the USB devices (keyboard and the mouse) had been unplugged for troubleshooting.
Update 3: Got another motherboard. Installed the CPU. It works. So, it looks like the motherboard was at fault. Now the PC won't recognize the old Windows system disk, but that's another story 🙂
Best Answer
The motherboard has suffered a catastrophic failure.
This was always the most likely possibility. The description of the CPU fan as rhythmically starting and stopping by itself almost always points to a failed motherboard (or one that needs a CMOS reset, which has already been ruled out.)
The failure rate for motherboards compared to CPUs is also vastly higher in general. I have had to replace over a dozen motherboards but have yet to come across a single CPU failure. CPU failures are rare as they have fewer potential points of failure than motherboards and almost always outlive them.
In terms of proof that it's the motherboard and not the CPU, the ASUS Z-97 manual actually describes how a failed CPU can easily be recognized by the illumination of a dedicated CPU POST-failure LED on the motherboard itself.
The manual also describes how if the motherboard does find an error with either the CPU, boot device, DRAM or VGA module, it will not reboot itself as is happening in this situation, but will simply keep the corresponding POST-failure LED steadily lit until the error is resolved. That's not what's happening here: it's continually restarting itself.
Everything points to a bad motherboard.