How to Test a Power Supply Unit (PSU) with a Digital PSU Tester
Summary
This article explains how to test an ATX computer power supply with a digital LCD power supply tester and how to read the results. Use it for new PSUs before installation, and when you suspect a PSU is causing a failure to power on, random shutdowns, or instability.
Applies To
- Standard ATX desktop power supplies, any brand or wattage
- Digital LCD PSU testers that have 24-pin, 4/8-pin CPU, 6/8-pin PCIe, SATA, and Molex inputs
Safety Notes
- Test the PSU outside the computer, or with all of its cables disconnected from the motherboard and components.
- Keep the PSU's power switch OFF while you connect or disconnect cables.
- Never open the PSU housing. It contains capacitors that can hold a dangerous charge even after it's unplugged.
Procedure
- With the PSU switch OFF, plug the PSU into a wall outlet.
- Connect the PSU's 24-pin motherboard cable to the tester's 24-pin slot. The tester needs this connection to power on and to read most values.
- Connect the CPU cable (4-pin or 8-pin EPS) to the tester's 4P/8P slot. Without it, the +12V2 reading shows LL.
- Turn the PSU switch ON. The tester should light up and show readings within a few seconds. Many testers beep once when they start.
- Record every reading and compare it to the table below.
- Disconnect the CPU cable, then test each PCIe (6/8-pin) cable the same way, one at a time.
- Test the SATA and Molex cables in their slots. Check that the voltage LEDs for each connector light up.
- For modular PSUs, test every cable you plan to use, because a defective cable can fail even when the PSU itself is fine.
- Turn the PSU switch OFF before you disconnect anything.
How to Read the Results
| Display Label | Source | Acceptable Range | Notes |
|---|---|---|---|
| +12V1 | 24-pin | 11.40 – 12.60 V | Main 12V rail |
| +12V2 | CPU (4/8-pin) or PCIe (6/8-pin) | 11.40 – 12.60 V | Shows LL when nothing is connected to that slot |
| +5V | 24-pin | 4.75 – 5.25 V | Powers USB, drives, and legacy devices |
| +3.3V | 24-pin | 3.135 – 3.465 V | Powers RAM, chipset, and slot logic. A display of 3.2 is normal |
| -12V | 24-pin | -10.8 to -13.2 V | Most testers don't show the minus sign, so 11.8 means -11.8 V |
| 5VSB | 24-pin | 4.75 – 5.25 V | Standby power that stays on when the PC is off |
| PG | 24-pin | 100 – 500 ms | "Power Good" delay before the PSU signals the motherboard to start |
The voltage ranges come from the ATX specification: ±5% for all rails except -12V, which allows ±10%.
Display Codes
- LL: Voltage is too low to measure, or nothing is connected to that input. If you get LL on a rail whose cable is connected, the PSU or the cable has failed.
- HH: Voltage is too high. The PSU fails the test.
- Blank screen or no power-on: The PSU didn't start. Check the wall outlet, the PSU switch, and that the 24-pin cable is fully seated. If all of those are fine, the PSU is likely dead.
- Continuous beeping or alarm: On most testers this means a reading is out of range. Check which value it is.
Pass / Fail Criteria
- PASS: Every connected rail is within its range, and PG is between 100 and 500 ms.
- FAIL: Any rail is out of range, PG is below 100 ms, above 500 ms, or missing, the tester shows LL or HH on a rail whose cable is connected, or the PSU doesn't power on.
- BORDERLINE: A reading right at the edge of its range (for example, 11.4 V on +12V). Retest. If the reading doesn't change, treat the unit as suspect, especially if the PC also shows instability.
Limitations
- This is a no-load test. It confirms the PSU starts and regulates voltage, but it can't show how the PSU behaves under a real workload.
- Low-cost testers are accurate to about ±0.1 V. Only readings clearly outside the ranges above are meaningful.
- If a PSU passes this test and the PC is still unstable, install the PSU and watch its voltages under load:
- Monitor the rails with HWiNFO64 (free).
- Run a combined CPU and GPU load test such as OCCT's Power test.
- Voltages should stay stable and within the ranges above. Motherboard sensors are approximate, so look for sudden drops or large swings more than exact values.