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1. First diagnostic step is to remove and reseat all connectors and components (not the CPU). That fixes most problems. The heat/cool cycle of operating a PC can lead to “thermal creep” where components work themselves loose.
2. In my experience, “sudden shutdowns” are usually heat-related – typically, the CPU. Next step would be to remove everything removable (hard drives, large flat cables, expansion cards) and blow the chassis/mother board out with high-pressure air. At work, I have a small compressor. Canned air won’t do it. Really get in and under everything you can. Pay attention to the insides of the power supply (through the slots) and the heat sink on the CPU. Put only the minimum memory back in and anything you need to get to the BIOS setup.
3. Boot to the BIOS setup screen and let it sit. If it shutdown there, you have eliminated all the components still not connected. At that point, I would remove and re-seat the CPU. Download the Dell manual. Remove the CPU with a static strap. Clean the surfaces and apply the correct amount of new thermal paste. Paste can dry out or flow over time. If that creates a hotspot on the chip, it could make it shutdown. Repeat the BIOS test.
4. If the PC still shuts down after step 3, then it’s likely a component is changing value over time. This is typical of electrolytic capacitors that have dried out. The paste-like electrolyte inside them contains water. After time or in high temps, they will out-gas; eventually, they dry out and lose their value. This throws whatever circuit they were part of out of spec. In a power supply, for example, they might lose the ability to regulate the voltage, causing it to drop below spec and cause the machine to reset or shutdown. On the circuit board, they are part of bus voltage regulation, clock timing circuits, and so on. You can try replacing the PS, but it’s more likely that the issue is on the MB.
5. If the PC does not shut down after step 3, then you can start adding components back to the PC – one at a time. Don’t try to boot, but let it sit at the BIOS screen each time. The idea is to see if an increased load on the power supply or MB is causing the issue. If it beings to shut down at some point, after adding components, you have about a 25% chance of it being the power supply, 75% chance MB.
6. If all is reconnected and still no shutdown, then boot to a USB drive or DVD. Let it sit at that screen. If it shuts down then, which would be strange, I’d suspect an intermittent failure of RAM or the RAM bus circuitry. At that point, it’s the MB.