Computer powers on but then shuts off after 3-4 seconds

[Gundam]

Currently building an el cheapo rig right now with a Foxconn barebones system and some extra parts I had laying around. The problem is that after I turn the computer on, I see the bootscreen, but then the computer shuts itself down after about 3-4 seconds. I've tried disconnecting everything but the cpu and ram, but still no go. I've also tried using a different power cable. My guess would be that there is something wrong with PSU. Would a faulty mobo cause this problem as well?

 

[Scott66]

Both can be possible reasons for your symptoms so first try a different power supply with CPU and RAM only and keep saying "Please, please, please" many times. Good luck

 

[stevty2889]

Could also be overheating, that will also cause a shutdown. Double check to make sure the heatsink is seated properly. Did you use thermal paste?