PC won't turn on? How to measure the ATX PSU PS-ON pin
Computer not starting? Learn how to diagnose if the issue lies in your motherboard's PS-ON signal or a dead power supply with this technical guide.
Summary
- The PS-ON signal is a logic level sent by the motherboard to command the ATX PSU to enable main rails.
- A missing 5V standby signal on the green wire indicates either a PSU failure or lack of standby voltage.
- Forcing the PS-ON pin to ground is the standard way to isolate the power supply from the motherboard logic.
- Sometimes the issue is deeper, such as when you need to test the chipset 3.3V line to allow booting.
- Stable voltage is not enough if the power supply fails to confirm with a proper Power Good signal .
Understanding the PS-ON Signal
When you press the power button on your PC case, a complex sequence of electrical events begins. The PS-ON signal, located on the green wire of the 24-pin ATX connector, acts as a logic switch. When idle, it maintains a positive voltage (usually 5V), kept alive by the PSU's 5VSB (Standby) rail. When the motherboard detects your power button press, it pulls this wire to ground (GND), signaling the PSU to release the main 12V, 5V, and 3.3V rails.
Why won't the PC turn on?
Often, the culprit is the motherboard, which fails to send this grounding pulse. If the PSU doesn't receive the ground signal, it stays in standby mode. Before blaming the PSU, isolate the variable: is the PSU ready but not called, or is it simply unresponsive? For advanced troubleshooting, always check for physical component damage, such as when you need to test tantalum capacitors for shorts on the motherboard.
How to measure the PS-ON signal
To test this, you will need a multimeter set to DC voltage. With the PSU plugged into the wall and the rear switch turned on, place the black probe on any black wire (GND) and the red probe on the green wire (PS-ON). The expected value is between 3.3V and 5V. If you find 0V, the PSU isn't producing standby voltage, pointing to an internal PSU fault. If the voltage is correct, the signal exists, but the motherboard is failing to pull the pin low.
The Load Test (Jump Start)
If your measurement shows the correct voltage but the PC still won't boot, you can force the PSU to start. With the 24-pin connector removed from the motherboard, use a small metal clip or wire to bridge the green pin (PS-ON) to any adjacent black pin (GND). If the PSU fan spins, the unit is functional. If it stays off, the PSU is faulty. If the PSU turns on but the system is unstable, verify the Power Good signal to ensure voltages are stable before the "OK" signal reaches the CPU.
Final Considerations
Hardware diagnostics require patience and a systematic approach. When identifying a non-starting PSU, your focus should be the primary circuit and the standby rail. If the PSU turns on during a jump test but fails when connected to the board, the issue is the PS-ON signal from the motherboard. Check for faulty power buttons or Super I/O control issues. Using a multimeter and this procedure, you can resolve 90% of your power-related doubts in minutes.