PSU Power Good PWR_OK Signal Delay and 5VSB Standby Rail Timing
Diagnose power supply startup handshake failures where +5VSB standby power is present but the PWR_OK (Power Good) signal fails to assert within 100-500ms.
Internal desktop parts run on harmless 12V DC voltage and cannot shock you. Simply unplug the wall power cable first, then touch the unpainted metal case frame to ground static electricity.
Common Symptoms & Tells
- Motherboard standby RGB lights illuminate but pressing power button produces an audible click and zero fan movement
- PC fan twitches for a fraction of a second then immediately shuts off
- System refuses to enter S0 running power state
Interactive Fix Checklist
0 of 5 steps completedSafety power-off and discharge
Shut down the PC, turn off the power supply rocker switch (O), and unplug the AC power cord from the wall outlet. Press the front power button once while unplugged to discharge residual capacitor energy before touching any internal components.
Verify Pin 9 (+5VSB purple wire) standby voltage
With plugged in and rocker switch on, test Pin 9 against ground with a multimeter. It must read between +4.75V and +5.25V DC. If absent or below 4.5V, the standby transformer is dead.
Measure Pin 8 (PWR_OK / Power Good gray wire) signal
The PWR_OK signal informs the motherboard chipset that all DC voltages have stabilized. ATX specification requires PWR_OK to rise to +5V within 100ms to 500ms after PS_ON# assertion. A delay outside this window causes the motherboard to abort startup.
Isolate 12V rail short circuits
Unplug the 8-pin EPS CPU power cable and all PCIe power cables. If the system turns on without them, a shorted MOSFET on the motherboard VRM or core is triggering protection.
Replace failing power supply unit
Degraded capacitors on secondary filtering stages often delay PWR_OK beyond the motherboard 500ms timeout window. Replace unit with a tier-A or tier-B rated power supply.
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