How to Test Power Good Signal on PSU with Multimeter
Learn how to diagnose Power Good signal failures in your power supply unit using only a digital multimeter on your workbench.
Summary
- The Power Good signal acts as a safety lock that releases the CPU only when all voltages are stable.
- The absence of voltage on the grey wire of the ATX connector typically indicates the PSU failed its self-test.
- Identifying failures in this signal prevents incorrect diagnostics on motherboards that fail to power on.
- Measurements must be performed with the PSU disconnected from the motherboard to isolate the fault.
- Checking Power On signals is vital to validate that the energy control system is operating correctly.
What is the Power Good signal and why does it matter?
In computer electronics, the 'Power Good' signal is the guarantee that the power supply unit (PSU) is delivering clean and stable electricity. Think of the PSU as a car engine; before allowing the processor to start working, the PSU performs an internal check-up on all voltage rails (3.3V, 5V, and 12V). If everything is within limits, it sends a positive electrical signal, usually 5V, through the grey wire of the 24-pin ATX connector. Without this release, the motherboard understands that the power is unstable and keeps the processor in a constant reset state.
Many junior technicians mistake a PSU that 'spins the fan' for a healthy one. However, the fan spinning does not guarantee that the internal circuits have finished the initialization process. This is where the multimeter becomes your main ally. By measuring the Power Good signal, you can discover in seconds whether a PC boot failure comes from the PSU or from another component, as detailed in our guide on how to test the motherboard power on circuit.
Preparing the workbench for testing
To test this signal safely, the first step is to isolate the PSU from the rest of the computer. Never attempt to measure Power Good while the PSU is connected to the motherboard, as voltage return or a short on the board could mask the result. You will need a digital multimeter set to DC voltage mode and, if necessary, a paperclip or jumper wire to simulate the power-on command.
The standard procedure involves closing the connection between the green wire (Power On) and any black wire (Ground/GND) on the 24-pin ATX connector. With the PSU powered and 'turned on' via the jumper, the multimeter should indicate a value close to 5V on the grey wire pin. If the value is zero or very low, the PSU has detected an oscillation or internal failure and has locked the output.
Step-by-step: measuring the Power Good signal
- Disconnect all PSU cables from the PC components.
- Identify the green wire (PS-ON) and a black wire (GND) on the 24-pin connector and insert a jumper between them to turn the PSU on.
- Place the multimeter's black probe on any black wire (GND) pin and the red probe on the grey wire (Power Good) pin.
- Check the reading on the multimeter: a value between 4.75V and 5.25V indicates the PSU is operating correctly.
- If the reading is 0V or close to it, the PSU has failed its self-test and requires immediate repair or replacement.
Interpreting results and trade-offs
If the multimeter shows 0V, you have confirmed that the PSU is not sending the ready signal. This can be caused by bulging capacitors, shorted semiconductors, or even a fault in the PWM controller integrated circuit. In high-quality power supplies, the signal rise time is monitored with precision. If your PSU is intermittent—meaning the PC boots sometimes but not others—it is possible that the Power Good circuit is operating at its limit, sending the signal only when the ambient temperature is ideal or the grid load is stable.
Always perform cross-measurements. Testing only the Power Good is insufficient without checking the 12V and 5V outputs under load, which we can compare later when analyzing more complex circuits, such as when verifying the clock circuit with a multimeter. Maintaining a methodical testing routine prevents unnecessary part swapping and isolates the diagnosis to the exact component at fault. To explore these architectural concepts further in production, see our related technical analysis on How to block ads on your network using Pi-hole.
Conclusion
Mastering the Power Good signal test transforms your workbench, allowing for rapid and precise diagnostics that separate power supply failures from logic board issues. By understanding that this pin is the 'gatekeeper' of the system, you move from being a part-swapper to a circuit diagnostician.
Always remember that in electronics, measurement is the only truth. If the PSU does not deliver the Power Good signal, it is not safe to power a modern processor. Apply these steps with care and use your multimeter as a primary safety tool before any deep intervention.