Testing a power supply with a multimeter tells you whether it's delivering the right voltage to your motherboard, CPU, and drives
A multimeter is a handheld meter that measures voltage, current, and resistance. When your PC won't start or shuts down unexpectedly, a faulty power supply is often the culprit. Testing it yourself takes about 10 minutes and costs nothing if you already own a multimeter—they run $15 to $40 new.
The test works because a power supply produces several different voltage rails: +12V for the CPU and graphics card, +5V for memory and some drives, +3.3V for chipsets, and -12V for legacy circuits. A multimeter can measure each one. If any rail reads significantly lower than it should, the power supply is failing and needs replacement.
You do not need to open the power supply itself. You test it by probing the connectors that plug into your motherboard and drives while the supply is running. This is safe as long as you follow the steps in order and never touch the metal prongs inside the connectors.
Key Takeaways
- Set your multimeter to DC voltage mode (marked V with a straight line) before you start, because AC mode will give you wrong readings.
- Test the 24-pin motherboard connector first, because it carries the main power rails your system needs to boot.
- A reading within 5 percent of the target voltage (for example, 11.4V to 12.6V for a +12V rail) means the supply is working; anything lower suggests failure.
- The black probe always goes on a ground pin (usually black wire), and the red probe goes on the voltage pin you are testing.
- If all voltages read low or zero, the power supply may not be receiving power at the wall, or the on/off switch may be off.
What you need before you start
Gather a digital multimeter (analog meters work but are harder to read), your PC, and a Phillips screwdriver. You do not need to remove the power supply from the case. Make sure your PC is powered off and unplugged from the wall for at least 30 seconds—this drains residual charge from the capacitors inside.
Plug the power supply back in but leave the PC off. You will turn on the power supply's physical switch (usually on the back of the case) to run the test. Do not press the power button on the front of the case; you only want the power supply running, not the motherboard.
If your power supply does not have a physical on/off switch, you will need to jump the power supply into test mode. This requires bridging two pins on the 24-pin motherboard connector with a paperclip—the green wire (PS_ON) and any black wire (ground). We cover this method in the troubleshooting section below.
Setting up your multimeter
Turn the multimeter dial to DC voltage mode. This is marked with a V and a straight line (not a wavy line, which is AC). Start with the 20V range if your meter has selectable ranges—this gives you enough headroom to read 12V safely without maxing out the scale.
Insert the black probe into the COM (common) jack and the red probe into the V (voltage) jack. If your meter has only two jacks, the probes are already assigned. Do not use the resistance or current jacks for this test.
Leave the meter on the bench in front of you where you can see the display while you work. You will need one hand to hold a probe steady and the other to write down readings.
Testing the 24-pin motherboard connector
Locate the 24-pin connector that plugs into your motherboard. It is usually near the top-left of the motherboard when you look at the case from the front. You do not need to unplug it—you will probe the pins from the back side of the connector while it is still seated.
Turn on the power supply's physical switch. You should hear the fan inside the power supply spin up. If it does not, the supply may be dead and you can stop here.
Gently separate the wires at the back of the 24-pin connector so you can see the individual pins. The connector has two rows of 12 pins each. Use the reference table below to find the pins you need to test.
| Voltage Rail | Target Voltage | Acceptable Range | Wire Color | Pin Location (24-pin) |
|---|---|---|---|---|
| +12V | 12.0V | 11.4V to 12.6V | Yellow | Pins 1, 2, 3, 5 |
| +5V | 5.0V | 4.75V to 5.25V | Red | Pins 4, 6, 7, 19, 20 |
| +3.3V | 3.3V | 3.14V to 3.47V | Orange | Pins 11, 12, 21, 23 |
| Ground | 0V | 0V | Black | Pins 8, 9, 10, 13, 14, 15, 16, 17, 24 |
Place the black probe on any black wire (ground). Hold it steady with one hand. With the other hand, touch the red probe to a yellow wire (the +12V rail). Read the voltage on the meter display and write it down. It should read between 11.4V and 12.6V. If it reads 12.0V exactly, that is ideal.
Repeat this process for the red wires (+5V rail) and orange wires (+3.3V rail). Test at least two pins on each rail to make sure the reading is consistent. If one pin reads 12V and another reads 10V on the same rail, the power supply is failing.
If any reading is more than 5 percent below the target (for example, below 11.4V for +12V), the power supply is not regulating voltage correctly and should be replaced. A reading that is slightly high (12.5V instead of 12.0V) is usually acceptable, but consistently high readings suggest the supply is aging.
Testing the 8-pin CPU power connector
Most modern motherboards have an 8-pin connector near the CPU socket that supplies dedicated power to the processor. This connector carries +12V on four pins and ground on the other four. Locate it on your motherboard—it is usually in the top-left corner near the CPU cooler.
With the power supply still running, place the black probe on a black wire in the 8-pin connector. Touch the red probe to a yellow wire. The reading should again be between 11.4V and 12.6V. If this rail reads significantly lower than the 24-pin +12V rail, the power supply's 12V circuit is degrading.
You do not need to test every pin, but testing at least one yellow and one black wire confirms the CPU is receiving stable power. If the reading is unstable or drifts while you watch, the power supply is likely failing.
Testing SATA and Molex connectors
If your PC has older hard drives or SSDs with SATA power connectors, you can test those too. SATA connectors carry +12V (yellow), +5V (red), and ground (black). Probe a yellow and red wire on a SATA connector the same way you tested the motherboard connectors. The readings should match what you saw on the 24-pin connector.
Molex connectors (the older four-pin connectors on some drives) carry +12V and +5V as well. Test them the same way. If SATA or Molex connectors read significantly lower than the motherboard connectors, the power supply's internal wiring or capacitors are failing and the supply should be replaced.
Turn off the power supply switch when you are done testing. Wait 30 seconds before unplugging anything to let the capacitors discharge safely.
What your readings mean
A healthy power supply delivers voltages within 5 percent of the target on all rails. This means +12V reads 11.4V to 12.6V, +5V reads 4.75V to 5.25V, and +3.3V reads 3.14V to 3.47V. If all your readings fall in these ranges, the power supply is working correctly and is not the cause of your PC problems.
If one or more rails read low—for example, +12V at 10.5V—the power supply is failing. This usually happens because internal capacitors are aging and can no longer hold a charge. A failing supply may work for a few minutes and then shut down as it overheats, or it may deliver unstable voltage that causes random crashes or data corruption.
If all rails read zero or very close to zero, the power supply is not receiving power at the wall outlet, the on/off switch is off, or the supply is completely dead. Check that the wall outlet works by plugging in a lamp. Check that the power cable is fully seated on both ends. If both are fine, the power supply needs replacement.
Troubleshooting when the power supply won't turn on
If the power supply fan does not spin when you flip the switch, the supply may have a safety feature that prevents it from running without a load. Some supplies require the motherboard to be powered on to set up the fan. In this case, you need to jump the power supply into test mode.
Locate the 24-pin motherboard connector. Find the green wire (PS_ON signal) and any black wire (ground) in the connector. With the power supply off, use a paperclip or a spare wire to bridge these two pins together. Turn the power supply on. The fan should spin. You can now test the voltages as described above. The green and black wires are usually next to each other, making this straightforward to do without unplugging the connector.
If the fan still does not spin after you bridge the pins, the power supply is likely dead and cannot be tested further. If the fan spins but all voltage readings are zero, the supply's internal circuits are broken and it needs replacement.
Frequently Asked Questions
Can I test a power supply while the PC is running?
No. Testing while the motherboard is powered on risks damaging the motherboard if you accidentally touch the wrong pins. Always power off and unplug the PC first, wait 30 seconds, then turn on only the power supply switch to run the test.
What if my multimeter does not have a DC voltage setting?
You need a multimeter with a DC voltage mode to test a power supply safely. AC mode will give you incorrect readings. If your meter only has AC, you will need to borrow or purchase a digital multimeter. Analog meters with a DC setting work but are harder to read accurately.
Do I need to remove the power supply from the case to test it?
No. You can test the power supply while it is installed in the case by probing the connectors from the back. This is faster and safer than removing the supply. Only remove the supply if you need to inspect it for physical damage like burned components or leaking capacitors.
What should I do if the readings are unstable and keep changing?
Unstable readings usually mean the power supply is failing. Capacitors inside are breaking down and cannot maintain steady voltage. Write down the highest and lowest readings you see. If the range is wider than 10 percent of the target voltage, the supply should be replaced. Unstable voltage can corrupt data and damage components over time.
Can a power supply fail the multimeter test but still work in the PC?
Possibly, but not for long. A power supply that reads low or unstable on a multimeter is degrading. It may work for a few days or weeks, but it will eventually fail completely. If your PC is crashing randomly or shutting down under load, and the multimeter shows low voltages, replacing the power supply will likely solve the problem.