This article explains how to test the power supply unit (PSU) in your computer with a power supply tester. Instructions apply specifically to the Coolmax PS-228 ATX power supply tester, but they also suffice for most other PSU testers with an LCD.

What to Know

  • Read PC Repair Safety Tips. Open case, unplug and organize power connectors. Check for proper voltage.Next, plug motherboard power connectors into power supply tester. Plug power supply into live outlet.Press and hold ON/OFF button on tester. Check power supply voltage tolerances and other criteria.

How to Test a Power Supply Using a Power Supply Tester

If you have some experience working on your computer and feel comfortable following instructions, this process should take about 30 minutes to complete. However, it may not be suitable for those who are new to computer maintenance. Before proceeding, be sure to read the safety tips. Once you have done so, you can begin the process.

Read Important Computer Repair Safety Tips before proceeding with testing a power supply unit. Working with high voltage electricity can be dangerous, and it’s crucial to prioritize safety when repairing a power supply. Do not neglect this step.

How to Test the Power Supply in a Computer

  • Turn off the PC, remove the power cable, and unplug anything else connected to the outside of the computer. Then, open the case.
  • To make the power supply test easier, move the disconnected and open case somewhere you can easily work with it, such as to a table or other flat, non-static surface.
  • Unplug the power connectors from every internal device inside the computer. 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 at one or more power connectors.
  • 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.
  • Group all the power cables and connectors together for easy testing. As you organize the power cables, pull them away from the computer case as much as possible to make it easy to plug the power connectors into the power supply tester.
  • Check to make sure that the power supply voltage switch located on the power supply is properly set for your country. In the U.S., this switch should be set to 110V/115V.
  • Plug both the ATX 24-pin Motherboard Power Connector and ATX 4-pin Motherboard Power Connector into the power supply tester.
  • Depending on the PSU that you have, you might not have a 4-pin motherboard connector but instead a 6-pin or 8-pin variety. If you have more than one type, only plug in one at a time along with the 24-pin main power connector.
  • Plug the power supply into a live outlet and flip the switch on the back. Some power supplies don’t have a switch on the back. If the PSU you’re testing doesn’t, plugging in the device is sufficient to provide power.
  • Press and hold the ON/OFF button on the power supply tester. You should hear the fan inside the power supply begin to run. Some versions of the Coolmax PS-228 tester don’t require that you hold down the power button, but others do.
  • Just because the fan is running doesn’t mean your power supply is supplying power to your devices properly. Also, some power supply fans don’t run when being tested even though the PSU is fine. You need to continue testing to confirm anything.
  • Confirm the LCD on the power supply tester is lit, and you see numbers in all the fields. 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 doesn’t light up at all, the power supply isn’t working properly; you need to replace it.
  • You’re just looking at the LCD screen at this point, so don’t worry about any other lights or voltage indicators not located on the actual LCD readout.
  • 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 100–500 ms, replace the power supply. The tester is designed to give an error when a voltage is out of range, but you should check yourself 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 14.
  • Turn off the switch on the back of the power supply and unplug it from the wall.
  • Plug in one connector to the appropriate slot on the power supply tester: a 15-pin SATA Power Connector or a 4-pin Molex Power Connector. Don’t 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.
  • Both of the motherboard power connectors that you connected to the tester earlier should remain plugged in throughout these tests of the other power connectors.
  • Plug in the power supply and then flip on the switch on the back if you have one.
  • 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.
  • 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. Only test one at a time, not counting the motherboard power connectors that stay connected to the tester the entire time.
  • When the testing is complete, turn off and unplug the power supply, disconnect the power cables from the power supply tester, and then reconnect the internal devices to power.
  • Assuming your power supply didn’t test as bad, 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.

Your PSU Is Good, But Your PC Won’t Start

Testing a power supply on a desktop computer using a power supply tester device is one of two ways to test one in a computer. There should be little doubt about whether your PSU is working properly after testing it with a PSU tester.

Turn off the PC, remove the power cable, and unplug anything else connected to the outside of the computer. Then, open the case.

To make the power supply test easier, move the disconnected and open case somewhere you can easily work with it, such as to a table or other flat, non-static surface.

Unplug the power connectors from every internal device inside the computer. 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 at one or more power connectors.

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.

Group all the power cables and connectors together for easy testing. As you organize the power cables, pull them away from the computer case as much as possible to make it easy to plug the power connectors into the power supply tester.

Check to make sure that the power supply voltage switch located on the power supply is properly set for your country. In the U.S., this switch should be set to 110V/115V.

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

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

Plug the power supply into a live outlet and flip the switch on the back. Some power supplies don’t have a switch on the back. If the PSU you’re testing doesn’t, plugging in the device is sufficient to provide power.

Press and hold the ON/OFF button on the power supply tester. You should hear the fan inside the power supply begin to run. Some versions of the Coolmax PS-228 tester don’t require that you hold down the power button, but others do.

Just because the fan is running doesn’t mean your power supply is supplying power to your devices properly. Also, some power supply fans don’t run when being tested even though the PSU is fine. You need to continue testing to confirm anything.

Confirm the LCD on the power supply tester is lit, and you see numbers in all the fields. 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 doesn’t light up at all, the power supply isn’t working properly; you need to replace it.

You’re just looking at the LCD screen at this point, so don’t worry about any other lights or voltage indicators not located on the actual LCD readout.

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 100–500 ms, replace the power supply. The tester is designed to give an error when a voltage is out of range, but you should check yourself 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 14.

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

Plug in one connector to the appropriate slot on the power supply tester: a 15-pin SATA Power Connector or a 4-pin Molex Power Connector. Don’t 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.

Both of the motherboard power connectors that you connected to the tester earlier should remain plugged in throughout these tests of the other power connectors.

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

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.

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. Only test one at a time, not counting the motherboard power connectors that stay connected to the tester the entire time.

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

Assuming your power supply didn’t test as bad, 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.

A test using a power supply tester isn’t 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.

However, there are several reasons a computer won’t start other than a malfunctioning power supply. See How to Fix a Computer That Won’t Turn On for a troubleshooting guide and more help with this problem.

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