Ok, so I am a first time computer builder. My setup is:
CPU: AMD FX-4170 4.2GHz Quad-Core
Mobo: ASUS Sabertooth 990FX AM3+
RAM: Corsair Vengeance 8 GB ( 2 x 4 GB ) DDR3 1600 MHz
PSU: Corsair Enthusiast Series 850-Watt
Graphics Card: Sapphire 11201-00-20G Radeon HD 7770 GHZ 1GB DDR5
Case: NZXT Crafted Series ATX Full Tower Steel Chassis
HDD: Seagate Barracuda 320G SATA
I have had this system up and running for a few days, and I came across some software issues (not important). I turned off and unplugged the desktop, but upon rebooting it the red CPU light came on. I found that I hadn't installed the standoffs behind my Mobo (yes, I know how obscenely stupid that was. don't hate). So I installed them. everything worked until the next time I unplugged it, after which the CPU light came on again. I don't know if this is a Motherboard issue, a CPU issue, or something else. Please help!
I would check all the major motherboard pins from the PSU to Mobo. In my time the spare 2x2 pin that you get which isn't required but usually is used sometimes just doesn't stick in well. If that doesn't work your processor may be overheating and shutting down quickly. Which would require repasting.
I can turn it off and on with no problems. But if I turn off the PSU or unplug it, it wont reboot properly. This can be temporarily solved by taking the CPU off and Re-installing it. Repasting might be a good idea. I don't think its overheating though. When it was functional I ran some pretty heavy stuff on it and it never hit 50C.