PC Buyer Beware!

Power supply problem: Computer won’t switch off, screen goes black, fans still spin, pressing the power button starts Windows again

PROBLEM: All of a sudden, when I shut my desktop PC down that is running Windows 7, the screen goes black, I can hear the fans working, but when I press the power button on the front of the case, it resets itself and Windows reboots. Nothing happens when I keep the button pressed to force it to shut down.

ANSWER: This is no doubt a power-supply problem. The motherboard instructs the power supply to shut down. The power button on the front of the case is connected to two pins on the motherboard. When those pins are shorted by pressing the button the motherboard sends a signal to the power supply, via the 24-pin ATX connector of the motherboard, which turns it on or off. Older motherboards power can require a 20-pin ATX connector from the power supply. If you are dealing with an old PC, check which connecter is being used before you upgrade the power supply, because it if only has a 24-pin connector, it can’t be used.

In this case, the power supply doesn’t seem to be getting the message to shut down even though the computer itself has shut down properly. When you press the power button to make the computer switch on, you are really only switching it on instead of resetting it.

Try unplugging the computer from the mains and remove the side of the case that gives you access to the motherboard. Find out where the 24-pin power connector that connects the power supply to the motherboard is on the motherboard and unclip it by pressing the plastic retention clip that holds it in place. Don’t try pulling the power connector out directly. It has to be unclipped by pressing the retention clip, which will allow the connector to be removed.

Now, with the power supply disconnected from the motherboard, plug the computer back into the mains supply, making sure that the power supply’s own on/off switch is turned on. If the power supply powers up and the fans start spinning, you have a faulty power supply, because it should not do so without receiving a signal from the motherboard, and you’ll have to replace it.

To replace the power supply, make a note of which cable goes where to the motherboard and the drives and perhaps to a PCI Express graphics card, etc., disconnect the old power supply, unscrew it from the back of the case (usually four screws secure it to the inside of the case), screw the new power supply into the case and attach the required cables to the motherboard, drives and perhaps the graphics card.

A modular power supply comes with cables that can be added as required; a standard power supply has its full range of cables already attached to it. A modular PSU is preferable because fewer cables – only those that are required – allows air to circulate more efficiently within the case.

Some PCI Express graphics cards, but not all, require one or two PCI Express power connectors connected to them from the power supply. Most motherboards also require an auxiliary 4-pin or 6-pin ATX power connector, which you should have noted where it goes when you disconnected the old power supply, but it if often forgotten. The computer won’t boot unless both power connectors are connected.

Note that the 4-pin or 6-pin connector can be located anywhere on the motherboard, not always beside the 24-pin connector.

The Build Your Own PC section of this site deals with installing all of the components, including a graphics cards and power supply (PSU).