Warning! You must unplug your ATX power supply from the wall before working inside the case. Troubleshooting PC failures starts with the power supply. In fact, it's easier to diagnose failures where the computer won't turn on at all as opposed to more intermittent issues. The flowchart below is one of 17 hardware diagnostic flowcharts included in my book, "Computer Repair with Diagnostic Flowcharts - Revised Edition." The expanded text is modified for this web published version, and the diamond symbols are linked to the text.
ATX PC Power Supply Diagnostic Flowchart
The first step in the troubleshooting process is simply determining if the power supply is coming on. You can usually hear those mechanical components in PC systems that make rotational noise when they are powered up. This usually includes the hard drive as it starts up the electric motor to keep the discs spinning, and some fan noise is pretty much a constant with modern computers. Your PC should also give a single beep if it passes its internal start-up diagnostic, and there are always status LED's to tell you the system is on, though some home PC builders don't bother connecting them. If your hearing isn't good, you can check to see if the power supply fan is creating a breeze. Monitors are powered independently, so unless you're looking at a notebook PC, a live screen doesn't indicate a working power supply. Return to Diagnostic Chart
If power isn't coming on, take the time to double check that the cord is plugged into a live socket and firmly seated in the back of the power supply. It's not necessary to own a DVM (Digital Volt Meter) to check your power outlet. Unplug the power supply cord from the outlet and plug a working lamp into the very same socket to test it. Don't assume that all the sockets in a power strip are working just because the power strip status light is lit. I'm always coming across power strips with one or more bad outlets. The power supply cord is basically bullet-proof, unless you cut through it with something, but if the PC gets moved or the cord gets kicked, it's easy for that cord to pull out a bit from the female socket on the power supply and still look like it's plugged in. Return to Diagnostic Chart
Check that the right voltage (110V/ 220V) is selected if the power supply has an exposed slide switch for voltage selection. This isn't going to be a problem for a PC that hasn't been worked on since the last time you used it, but if you've installed a replacement power supply, it's always possible it arrived on the wrong setting. The small red slide switch is recessed in the back of the power supply below the power cord so it's rare for it to get accidentally switched, even by rough handling. You should always unplug the power supply cord before changing the voltage because modern ATX power supplies are always live when plugged in. While it's not recommended that you experiment, if you plug in the power supply with with the switch on 220V in a 110V country like the U.S., it may still be OK when you correct the voltage. But if you power on a supply set for 110V in a country with a 220V distribution, you'll probably blow the power supply fuse (at the very least), and potentially damage the supply and the connected components. Return to Diagnostic Chart
If you press in the power switch on your system and it doesn't shut off the PC, that's how ATX systems are supposed to work. The power switch is programmable and the action can often be defined in CMOS Setup. The normal setting for PC power switches makes you hold the switch in for three to five seconds to shut down the system. Pressing the switch for a shorter duration might put the system in sleep mode or wake it up from hibernation, important options for power conservation. If Windows fails to turn off the power when you select "shut down", it's usually due to a corrupted file or bad setting in the operating system, and the first...