If you blame the computer for not being on, you may have this problem. First check whether there are any error messages when powering up such as CPU temperature exceeds its limit, watch out for IRQ conflicts (such as PCI slot errors) and memory errors. If there are no such error messages, it means you have a motherboard and power supply failure (this is usually caused by static electricity — be sure to use an anti-static pad when handling computer parts). In this case, check if the power supply produces 12 volts and 5 volts using a digital voltmeter. If not, replace the power supply. If it exists, check if the motherboard has 12 volts and 5 volts. If not, replace the motherboard.
If you have forgotten to check whether there are any error messages when powering up, or if your computer does not display anything on its screen at all (power supply failure), you need to do further checks;
PS/2 ATX Motherboards have a small two-pin diagnostic connector. You can use a voltmeter to check if the voltage output is normal (on the motherboard, there’s a 2-pin “D” diagnostic port). If it does not produce 5 volts and 12 volts in this way, you should replace the power supply.
If your computer fails to start up normally, you need to look for memory errors; Run Memtest86. It runs through all the system RAM in sequence and reports any errors found during each pass (Memtest86 information). When running Memtest86, do NOT interrupt it or turn off your PC when errors are detected — that might result in BSOD or make things worse! Go grab some coffee while waiting for it to finish.
Whether the motherboard or power supply is faulty?
It’s easy to determine which one is faulty; I’ll give you an example of how to identify which one has failed. But before that, you need to know certain terms related to motherboards and power supplies: +5 volt: The 5 volts from a modern ATX computer power supply is usually provided by either a ‘standby’ or ‘always on’ linear regulator. The standby type can be identified by two green wires coming out of it; the always-on type by four black or red wires. +3.3 volt: Sometimes the 3.3 volts come from an onboard voltage regulator and sometimes from the main switching regulator output (the 3.3 volts is usually taken from the 5 volts using a voltage divider). +12 volt: Is also provided by either standby or an always-on linear regulator.
After you check whether there is any issue related to CPU, memory, PCI slot, and IRQ conflicts, unplug all the components one by one except for the CPU and RAM. If your computer stops working after disconnecting something – no beeps (if it was caused by RAM problem), no POST screen (if it was caused by motherboards’ problem) — it’s the power supply that caused the problem. In this way, you can determine whether a failure is caused by the motherboard or power supply.
So far I’ve explained how to identify which component has failed. But if you don’t have any knowledge about computer hardware and just want to know what kind of part causes failure, I suggest you buy a pre-packaged diagnostic kit from your local computer store (such as CPU/RAM/mobo). It usually costs 2~3 times more than normal kits, but they are easy to use with detailed instructions — so I recommend them for novice users who don’t know how to do diagnosis using software tools.
Run the Diagnostic programs. For Windows, you can use Third-Party Diagnostics to run all sorts of hardware tests such as Hard Drive Controller Tests, which actually is not a motherboard test (they are usually bundled with Motherboard Drivers CD) — note that they are designed for graphical user interface and works only on Windows GUI.
If you feel confident about your diagnostic skills, or if there’s any software tool that enables you to check detailed error messages produced by the CPU/RAM/mobo, go ahead and start using them.
Otherwise, just follow my simple instructions below:
1) Connect your monitor to the motherboard’s onboard VGA port instead of PCI slot; If you have DVI or HDMI output, connect it to one of the motherboard’s onboard video ports (or use a converter cable).
2) Remove the case cover and connect a known good power supply to your computer.
3) Turn on the power.
4) Check whether your monitor displays a POST screen; if it doesn’t, turn off the switch immediately.
5) If you can see POST messages, leave them there for at least 30 seconds.
If there are any beeps indicating hardware issues (check out POST Beep Codes page), try to identify which part has failed using the table in How To Know Whether CPU/RAM/mobo Has Failed? the section below or run software tool that comes with the diagnostic kit.
If not able to find out what causes failure, try this:
6) Press [Del] key to enter BIOS Setup.
7) Go to Advanced > CPU Configuration and disable CPU Internal Cache.
8) Run the diagnostic program and see whether your computer can boot into Windows.
9) If yes, then you’ve found out that it’s the RAM/CPU which has failed – otherwise check the top-most memory module (the one closest to the CPU) whether it is properly seated on the motherboard (some motherboards require a certain number of pins to be inserted in holes).
10) After confirming which part has failed, try replacing it with a new or known good spare component. If there are any questions related to this article, feel free to ask me on my blog.