Too much thermal paste. (It doesn't seem to have gotten anywhere it shouldn't.)
Sat the motherboard on top of an anti-static bag at one point. (Stumbled across a post saying not to do that while searching for tips on this CPU problem.)
I'm wondering if there's anything else I can do to diagnose this. The CPU was ~$300, the motherboard ~$125. I think it will cost at least $60 to have a PC repair shop test the components. I don't want to risk breaking a second PC by mix-and-matching the parts myself. I'd like to identify the problem and resolve it for the least amount of money--maybe everything works, even, and I've just overlooked something.
The sticky post here says to plug in both the 20/24-pin and 4-pin power supply cords.
If you have only a 20/24-pin cord plugged in, could that cause the CPU and CPU fan to not get power?
All other fans, including the fan on the graphics card, all power on. All other LEDs light up. But the CPU fan only moves for a half-second on power up, and the LED on the CPU fan doesn't come on. I tried powering on with the CPU fan unattached and the CPU did not become warm. (I know it's a bad idea to leave the PC on for any more than a few seconds with no heatsink/fan attached. Let me know, though, if this diagnostic step is unhelpful. I've read that the CPU quickly overheats--even within 10 seconds--when powered on with no heatsink/fan, so I reasoned that if the CPU doesn't even get warm then it must not be getting power at all.)
Motherboard: GIGABYTE GA-EP45-UD3P LGA 775 Intel P45 ATX Intel Motherboard
CPU: Intel Core 2 Quad Q9650 Yorkfield 3.0GHz 12MB L2 Cache LGA 775 95W Quad-Core Desktop Processor
PSU: ATX-CW500P4 500W TurboLink ATX 12V
I tried plugging the 4-pin cord from the PSU into the righthand side of the 8-pin port on the motherboard, which the manual seems to indicate is the right way to plug it in (and it's the only way the cord will fit into that port). When powered on then, the CPU fan moves for a half-second as before, and then EVERYTHING goes out--all lights and LEDs. This is with the CPU fan not screwed into place but just sitting on top of the CPU. (I don't see how it could make a difference, but maybe it does.)