I bought a used desktop computer and connected it to power, a monitor, and Ethernet. A small light beside the Ethernet port comes on, so the system appears to receive standby power, but pressing the case power button does nothing: no fans, case lights, keyboard or mouse lights, and no display signal. What should I check to figure out whether the problem is the power button, wiring, power supply, or another component?
5 Answers
If the case button or its wiring is faulty, you can test the motherboard by briefly touching the two power-switch pins with a screwdriver. Disconnect the case switch connector first and use the motherboard manual to identify the correct pins. If it starts that way, the button or cable is the likely problem.
Start with the simple checks: make sure the power supply’s rear switch is set to the on position, try another power cable or wall outlet, and open the case to check that the motherboard’s 24-pin and CPU power connectors are firmly seated. Also verify that the case power-button connector is attached to the correct motherboard pins. The motherboard manual will show the exact front-panel header layout.
Don’t hold the power button for normal startup—a quick press is all that’s needed. Also note when the monitor shows “no signal.” If it displayed that before pressing the button, the computer may not be starting at all; if the message appears only after pressing it, the button may be working and the system could be failing during power-on. Once the fans or lights come on, troubleshooting the display becomes a separate step.
Before swapping parts, ask whether it was demonstrated working when you bought it and whether it was moved or stored afterward. Transport can loosen RAM, power connectors, or other components. With the system unplugged, reseat the RAM and internal cables, look for anything obviously loose or shorting against the case, and then try starting it with only the essential components connected. If it never worked before purchase, there may be a failed component rather than a simple connection issue.
Since the Ethernet light only indicates standby power, it doesn’t prove that the power supply can start the computer. If the wiring looks correct, test with a known-good power supply or have the current one tested. Avoid opening the power supply itself; mains capacitors can remain dangerous. If a replacement PSU doesn’t help, the motherboard or a short may be at fault.
A PSU can have standby power for the network port and still fail when the system tries to turn on, so that Ethernet light isn’t enough to rule it out.
I tried the power-supply switch in both positions several times, but I’ll check the internal connectors and front-panel wiring next.