How to Use a Power Supply Tester to Test a PSU

Photo of an Innoo Tech Power Supply Tester
Innoo Tech Power Supply Tester. © Innoo Tech

Testing a power supply using a power supply tester device is one of two ways to test a power supply in a computer. There should be little doubt about whether your PSU is working properly or not after testing it with a power supply tester.

Note: These instructions apply specifically to the Coolmax PS-228 ATX Power Supply Tester (available from Amazon) but they should also suffice for nearly any other power supply tester with an LCD display that you might be using.

Important: I'd rate this process as difficult but don't let that sway you from trying. Just follow the directions below carefully, most importantly #1.

Time Required: Testing a power supply with a power supply tester device will usually take around 30 minutes or a bit more if you're new to this sort of thing.

How To Test a Power Supply Using a Power Supply Tester

  1. Read Important PC Repair Safety Tips. Testing a power supply unit involves working around high voltage electricity, a potentially dangerous activity.

    Important: Do not skip this step! Safety should be your primary concern during a power supply test with a PSU tester and there are several points you should be aware of before beginning.

  2. Open your case: turn off the PC, remove the power cable, and unplug anything else connected to the outside of the computer.

    To make your power supply test easier, you should move your disconnected and open case somewhere you can easily work with it, like on a table or other flat and non-static surface. You won't need your keyboard, mouse, monitor, or other external peripherals.

  1. Unplug the power connectors from each and every internal device in side the computer.

    Tip: An easy way to make sure each power connector is unplugged is to work from the power cable bundle coming from the power supply. Each group of wires should terminate to one or more power connectors.

    Note: It's not necessary to remove the actual power supply from the computer nor should you need to disconnect any data cables or other cables not connected to the power supply.

  1. Group all of the power cables and connectors together for easy testing.

    As you're organizing the power cables, I recommend rerouting them and pulling them away from the computer case as much as possible. This will make it as easy as possible to plug the power connectors into the power supply tester.

  2. Check to make sure that the power supply voltage switch located on the power supply is properly set for your country.

    In the US, this switch should be set to 110V/115V. You can reference the Foreign Electricity Guide for voltage settings in other countries.

  3. Plug both the ATX 24 pin Motherboard Power Connector and ATX 4 pin Motherboard Power Connector into the power supply tester.

    Note: Depending on the power supply that you have, you may not have a 4 pin motherboard connector but instead have a 6 pin or 8 pin variety. If you have more than one type, only plug one in at a time along with the 24 pin main power connector.

  4. Plug the power supply into a live outlet and flip the switch on the back.

    Note: Some power supplies do not have a switch on the back. If the PSU you're testing does not, simply plugging the device in is sufficient to provide power.

  5. Press and hold the ON/OFF button on the power supply tester. You should hear the fan inside the power supply begin to run.

    Note: Some versions of the Coolmax PS-228 power supply tester do not require that you hold down the power button but others do.

    Important: Just because the fan is running does not mean that your power supply is supplying power to your devices properly. Also, some power supply fans do not run when being tested with a power supply tester even though the PSU is fine. You'll need to continue testing to confirm anything.

  1. The LCD display on the power supply tester should now be lit and you should see numbers in all the fields.

    Note: The motherboard power connectors plugged into the power supply tester support the entire range of voltages that your PSU can deliver, including +3.3 VDC, +5 VDC, +12 VDC, and -12 VDC.

    If any voltage reads "LL" or "HH" or if the LCD screen does not light up at all, the power supply is not working properly. You'll need to replace the power supply.

    Note: You're just looking at the LCD screen at this point. Don't worry about any other lights or voltage indicators not located on the actual LCD readout.

  1. Check Power Supply Voltage Tolerances and confirm that the voltages reported by the power supply tester are within approved limits.

    If any voltage is outside of the range shown, or the PG Delay value isn't between 100 and 500 ms, replace the power supply. The power supply tester is designed to give an error when a voltage is out of range but you should check yourself just to be safe.

    If all the reported voltages fall within tolerance, you've confirmed that your power supply is working properly. If you'd like to test the individual peripheral power connectors, continue testing. If not, skip to Step 15.

  2. Turn off the switch on the back of the power supply and unplug it from the wall.

  3. Plug in one connector to the appropriate slot on the power supply tester: a 15 pin SATA Power Connector, a 4 pin Molex Power Connector, or a 4 pin Floppy Drive Power Connector..

    Note: Do not connect more than one of these peripheral power connectors at a time. You probably won't damage the power supply tester doing so but you won't be accurately testing the power connectors either.

    Important: Both of the motherboard power connectors that you connected to the power supply tester in Step 6 should remain plugged in throughout these tests of the other power connectors.

  4. Plug in your power supply and then flip on the switch on the back if you have one.

  5. The lights labeled +12V, +3.3V, and +5V correspond to the voltages being delivered through the connected peripheral power connector and should light up appropriately. If not, replace the power supply.

    Important: Only the SATA power connector delivers +3.3 VDC. You can see the voltages delivered by the different power connectors by looking at the ATX Power Supply Pinout Tables.

    Repeat this process, beginning with Step 11, testing the voltages for the other power connectors. Remember, only test one at a time, not counting the motherboard power connectors that stay connected to the power supply tester the entire time.

  6. Once your testing is complete, turn off and unplug the power supply, disconnect the power cables from the power supply tester, and then reconnect your internal devices to power.

    Assuming your power supply tested good or you've replaced it with a new one, you can now turn your computer back on and/or continue troubleshooting the problem you are having.

    Important: A power supply test using a power supply tester is not a true "load" test - a test of the power supply under more realistic usage conditions. A manual power supply test using a multimeter, while not a perfect load test, comes closer.

Did the PSU Tester Prove Your PSU Good but Your PC Still Won't Start?

There are several reasons a computer won't start other than a malfunctioning power supply.

See our How To Troubleshoot a Computer That Won't Turn On troubleshooting guide for more help with this problem.