I'm troubleshooting an older iBUYPOWER prebuilt that intermittently becomes completely unresponsive after AC power is removed. The original Gigabyte B560 DS3H AC motherboard was diagnosed as faulty, so I installed a replacement board and verified the 24-pin ATX and CPU power connections. Initially, the replacement board showed no response at all—no fans, LEDs, or other signs of power—even when I manually jumped the case power-switch pins.
While disconnecting nonessential cables such as front audio, an extra system-fan connection, and the front USB header, the fans unexpectedly began spinning slowly and the system powered on. After reconnecting everything, I was able to start and shut it down several times normally, although I did not connect a monitor to confirm POST.
After shutting down, switching off the PSU, unplugging the AC cable, waiting, and reconnecting everything, the same completely dead condition returned. I tested both the original PSU and a known-good PSU with the same result. I also tested the replacement motherboard outside the case on a nonconductive surface with only the CPU, cooler, RAM, and PSU connected. Jumping the power pins still produced no response, and removing the RAM made no difference. The LGA1200 socket pins look normal.
What could cause a motherboard to work temporarily but become completely unresponsive after AC power is removed? Is there anything else I should test, such as fully discharging residual power or checking for a short or damaged connection?
1 Answer
Before testing again, disconnect the PSU from AC, switch it off, and press the case power button several times to discharge any remaining energy in the system. Also, avoid plugging or unplugging internal headers while the PSU is still connected to AC, even if the computer appears to be off, since standby power may still be present and components or headers can be damaged.
The system was not actively running when I disconnected the cables, but I had not tried repeatedly pressing the power button with the AC cable removed. I’ll try that discharge procedure before continuing, while making sure the PSU is fully disconnected first.