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When a Computer Power Supply Fails

written by: Leo Ponton•edited by: J. F. Amprimoz•updated: 5/19/2011

Like everything else, computer power supply units (PSUs) eventually fail. Identifying the PSU as the problem in a broken computer is not always that easy. We explain how to diagnose and correct power supply problems.

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    Identifying the Problem

    I’m assuming you have some technical knowledge and are competent working on computer electronics, otherwise take your computer to a repair shop. Take note of the Terms and Conditions of this website and the disclaimer on the following page.

    The most obvious sign of PSU failiure is that the computer will not switch on. Try a different power lead (the cable connecting the computer to a power outlet), and a different wall socket (directly into the wall, not via an extension or multi socket as these may be defective). If you can’t hear the fan going round at the back of the computer, it’s almost certainly the PSU. If there is a switch on your PSU, make sure it is on. If that isn't the problem, switch the PSU off, disconnect the power cable, open the case up and unplug and replug the motherboard power connectors. Sometimes they can get dirty or oxidised, and this should renew the connection and gives you a chance for a visual inspection.

    Erratic behaviour at startup - multiple beeps, won’t boot every time, frequent crashing - can all be symptoms of a failing PSU, but may also point to other problems such as a defective motherboard or graphics card. Motherboard manufacturers usually supply a lookup chart for the beep codes that might point you in the right direction.

    Grinding or squeaking are usually indicative of a worn fan. If it is the fan in the PSU, replace the PSU. I’ve replaced the fans before, but it can be awkward and if the fan hasn’t been cooling, the PSU has been overheating. It will fail soon. Also, the inside of a power supply is a very dangerous place, able to contain high voltages despite having been disconnected for signifcant periods.

    If you have and can use a multimeter, you can test the PSU. Disconnect the power supply from the motherboard and drives, etc. To switch it on, you’ll need to briefly short out the power switch terminals. They’re usually the green and any black (ground). The excellent Russian website, has a guide for the expected voltages at each pin ( If any pins fail the test, ditch the PSU and buy a new one.

    An inexpensive power supply tester is much easier to use for those who aren't electrically inclined.

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    Misleading symptoms

    Unfortunately, the symptoms may point you in the wrong direction. A game that crashes might be bad software, overheating graphics card, bad memory guessed it...a slowly dying PSU. It may, as suggested earlier, be a faulty motherboard. Sometimes, you just have to change it and see.

    If it’s a few years old and you’ve been upgrading your hardware, it is probably under-rated now anyway. You can find out how many watts your power supply needs here. Power supplies aren’t terribly expensive, so if you mis-diagnose it isn’t the end of the world.

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    Replacing the PSU

    It’s not usually very difficult to change the power supply. Before opening the computer case you’ll need to disconnect the mains supply - switch the PSU off if it has a switch and pull the power cable out. Disconnect everything else as well - monitor cable, keyboard, mouse - the lot.

    Open up the case and carefully unplug all of the power cables from the disk drives, CDs etc. and also the motherboard connectors which are ‘latched’, i.e. you’ll have to depress a small catch to release them (the drive connectors can be a little stiff, but they’re not latched). Note that on some motherboards only the longer connector is used. Check again that everything is disconnected by following the wires that exit the power supply.

    Using a phillips screwdriver remove the screws that fix the PSU to the case. When working on PCs, I use a magnetised screwdriver which makes three-handed jobs like this a lot easier. You can magnetise a screwdriver by stroking a strong magnet along the length of the shaft in one direction only. The more strokes you do the stronger the magnetic effect will be. Best to keep it away from your hardrives if you do this. As you undo the last couple of screws you should support the PSU so that it doesn’t fall into the case, potentially damaging the motherboard, RAM etc. With all of the screws removed, carefully lift the PSU away from the case. Be careful not to lose any of the screws inside the case!

    Replacement is basically the reverse of the above. While holding the new PSU in place, insert and tighten the screws (the new PSU may have a different number of fixing screws). Reconnect the motherboard connectors ensuring that they are the right way round and that the catch engages. The design of the connector makes it almost impossible to connect the wrong way. Then the connectors for the drives - again they are shaped in such a way that they can only be put in the right way round. You can see detailed instructions and pictures of how to install a PSU here.

    Now close the case, reconnect all the cables and plug in the power lead. Make sure the PSU is switched on if it has a switch. Your computer should now switch on normally. If it doesn’t, first check all of the external connections (monitor etc.). If it still doesn’t work, it is probably the motherboard connection.

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    Disclaimer of Liability:

    With respect to the information in this article, neither the publisher nor the author makes any warranty, express or implied, including the warranties of fitness for a particular purpose, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.