My friend and I were repairing a pc the other day that wouldn't boot up. It would turn on for a few seconds then shut back down again. Eventually we got it working by replacing the power supply. Then we got the steady amber light (the pc is a Dell Dimension e521), so we reset the bios and that got it working for a bit, but then it happened again. So we removed the CPU's heatsink, and removed and replaced the CPU. But the CPU was stuck to the heatsink from dried thermal paste, so we had to remove the CPU from the heatsink, which was fine, but we had no thermal paste to put back on. So we just put the CPU back into the slot and reattached the heatsink. Then, miraculously is WORKED! It worked for about 1 hour then shut down again. We then realised we hadn't screwed in the screws on the heatsink, it was just sitting there. So we did that and then it worked fine again. We had it running for about 2-3 hours and was in the process of installing Windows XP, when it turned itself off, but when we turned it back on it was fine. It then kept doing that and eventually we turned it on and it gave us the deadly amber light on the front panel. So we took the Bios battery out over night and put it back in the morning, but it didn't work. I then tested the Graphics card on another PC and it works fine, the RAM is fine, and the PSU is fine. So we have came to the conclusion that is the the CPU or the motherboard. We reckon it is the CPU because of the following reasons. When we had it working normally it shut down a couple of times and said shut-down because of thermal problems, and the CPU fan would sometimes go ballistic. Also, we had no thermal paste applied so we reckon we may have overheated and fried the CPU. But the annoying thing is, when we turned it on with the amber light and left it running, the CPU would heat up slightly but it took ages and it didn't get that hot (this is also without heatsink and fan).
If anyone has an answer it would be greatly appreciated. Sorry about the long post, but I want to make sure I have described everything in detail so you guys know the exact problem, which then could help me get my PC going. Thanks in advance