My computer powered off the other day on its own, and now when I push the power button, nothing happens. My assumption would naturally be that the power supply is done (possibly well done) but is there any good way to test this before I buy a new one?

  • 2
    Please tell me your Hard drives were "hot swappable"? Commented May 1, 2011 at 22:27
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    Power supplies have a short-circuit cutout which resets after being disconnected for 30 seconds or so. Disconnect the mains supply to the computer completely and wait for at least 30 seconds. Reconnect it and try again.
    – Majenko
    Commented May 1, 2011 at 23:58
  • 1
    I can tell you that my hard drives survived. I can't tell you that they are hot-swappable (or else I'd be lying).
    – Sam Hoice
    Commented Nov 17, 2011 at 23:27

5 Answers 5


Unplug the power supply from any of the components inside the computer (or just remove it from the computer completely).

USE CAUTION HERE (Though you'd only be shocked with a max of 24 volts)

  1. Plug the power supply into the wall.
  2. Find the big 24-ish pin connector that connects to the motherboard.
  3. Connect the GREEN wire with the adjacent BLACK wire.
    (If you're colorblind, the green wire sits between three black wires on one end and one more on the other)
  4. The power supply's fan should start up. If it doesn't then it's dead.
  5. If the fan starts up, then it could be the motherboard that's dead. You can use a multimeter to check if there is power output from the power supply.

enter image description here

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    +1, I've done this a few times. Looks like this tallahasseelights.com/tech/fm_transmitter/fm-004.jpg
    – hyperslug
    Commented Aug 17, 2009 at 17:58
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    Yup, I used to do this all the time when I worked at GEEK SQUAD. The managers didn't like it because the managers though the inside of the computer was ugly and customer's didn't want to see it. I did it anyway because it's a quick check to see if the mobo/power supply is dead and then they don't have to check in their computer for a week.
    – Grant
    Commented Aug 17, 2009 at 18:03
  • @ Grant - And that is why you no longer work for GEEK SQUAD... your'e too good for them.
    – J. Polfer
    Commented Aug 19, 2009 at 13:09
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    Fun fact: an ATX PSU is actually 2 psus in one case : the big one which can be turned on and off and a small 5V one that's always on and provides standby power. It is possible for only the standby part to fail, which will make the main PSU work when hotwired, but will still be useless in a PC, as without standby power the mobo could not turn the main part on.
    – Agent_L
    Commented Jul 15, 2016 at 14:18
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    @OhadSchneider Either is fine, both black wires go to the same place.
    – Grant
    Commented May 24, 2018 at 16:17

Most well-stocked geek-stores sell a "power-supply tester" that has all the appropiate connectors to plug each part of your PSU into, with spiffy LEDs indicating status of the various rails, connectors for IDE/SATA/floppy power cables, etc. They run ~$20 US.

enter image description here


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    I recommend this if someone is not confortable with a multimeter or connecting wires as suggested. Otherwise, Grant's suggestion is the free way to do it without another computer to test it on.
    – Troggy
    Commented Aug 17, 2009 at 18:47
  • It's much quicker using those, they show you in a second if all of the connectors are fine; doing it by hand takes much longer, and can be a bit fiddly with something like a SATA power connector.
    – Dentrasi
    Commented Aug 17, 2009 at 19:14
  • Great point! I should have guessed that there was a device for testing this. If I had this problem more often, I'd be more inclined (and I'd be sad).
    – Sam Hoice
    Commented Aug 25, 2009 at 16:25
  • Does that mean I should be sad for having need for one?
    – Adrien
    Commented Aug 25, 2009 at 23:45
  • Should I be sad for not even knowing it existed? Commented Oct 18, 2013 at 15:27

You could try using a multimeter to check the +/-5V and +/-12V rails. Due to the switching nature of the power supply, you'll want to put a resistor in series with your probes while measuring.

  • Getting/learning how to use a multimeter with electronic products is a good thing to do, and part of becoming superuser-awesome. You can pick up a multimeter suitable for this work for less than a trip to McDonalds, and sometimes free after rebate. That said, the above answers are better. But don't forget the multimeter and its awesome.
    – J. Polfer
    Commented Aug 17, 2009 at 20:56
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    Using a multimeter can sometimes give a false positive too. In my experiance, the most common failure mode for a PSU is the capacitors on the primary side. In this case the multimer shows the voltage to be just fine in an open circuit condition, but as soon as you draw any current, it drops to an absurdly low level. So you need a multimeters to measure voltage, and a resistance as a linear load and an accurate estimate of the loading characterstics of the mobo in order to draw any conclusions. Commented Sep 1, 2009 at 11:30
  • @Vaibhav - good point, I've added that to my answer.
    – J. Polfer
    Commented Sep 1, 2009 at 21:26
  • @VaibhavGarg - what would you use as a resistance to test with? Just a standard resistor or ..., what value of resistance is enough to test against? Thanks.
    – pbhj
    Commented Jan 15, 2015 at 14:43
  • @pbhj A resistance of high enough wattage and value so that it does not burn out, and the value low enough to draw a meaningful amount of current. Commented Jan 16, 2015 at 3:45

To test the power supply unit, follow these instructions from Corsair in the Power Supplies FAQ:

My Machine doesn’t start up when I hit the power button, is the PSU faulty?

Disconnect everything from your PSU except for one single fan that should be connected directly to one of the Molex connectors. Then, get a small piece of wire, paper clip, or suitable object and short the green pin and a black pin on the 24-pin connector on the power supply. The voltage present is a very low signaling voltage so no worries of being shocked. Your PSUs fan should spin along with the fan you have connected to it. If this is the case, your PSU may not be receiving the power on signal from your motherboard and you should consider other causes of the problem you’re having.

When a computer is turned on, the motherboard electronically connects the green pin (power on signal) to any one of the black connectors (ground). This signals the PSU to start supplying power. By shorting these two connectors, you are testing the PSU to determine if it can turn on when signaled to do so. If this doesn't work, then the PSU has probably failed. More information what each pin does on an ATX power supply unit is available on Wikipedia.


I have used one of the testers which showed a faulty supply to be good. The problem is the tester doesn't put a proper load on the PSU.

I replaced the supply and that fixed the problem. The voltage is OK if no load is applied. I serviced yachts for many years with 12 volt lighting. Corroded connections due to salt water are very common. Customers would return a new bulb and claim it was bad. I had to explain that 12 volts would show on their multimeter with the bulb removed, but this did not prove the bulb was bad. The corroded connection limited the current. Same principle.

  • The information you wrote is certainly useful but it is not written as an answer to the question. Could you please edit your text so that it contains an answer to "How to test a power supply?" Commented Oct 18, 2013 at 22:41

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