A desktop workstation powers on with fans spinning at maximum speed continuously, but it fails to POST, displays no video output, produces no diagnostic beep codes, and fails to shut down even when the power button is held for 10 seconds. A technician uses a multimeter on the 24-pin ATX power connector and verifies that the +3.3 VDC, +5 VDC, and +12 VDC main rails are within normal operating tolerances. However, testing the Gray wire (PWR_OK) indicates a constant reading of 0 VDC. Which of the following is the MOST likely root cause of this failure?
- AThe BIOS/UEFI firmware corrupted during startup, preventing the system from initializing POST routine check routines or generating error codes.
- BThe system RAM modules are installed in non-matching memory channels, preventing dual-channel memory initialization.
- The power supply unit is defective because it fails to assert the Power Good signal required for the motherboard chipset to release the CPU from reset.Cevap
- DThe CPU thermal throttling mechanism engaged due to inadequate contact with the heat sink, forcing immediate core shutdown.
Cevap
The power supply unit is defective because it fails to assert the Power Good signal required for the motherboard chipset to release the CPU from reset.
The correct answer identifies that the power supply unit is failing to assert the +5 VDC Power Good (PWR_OK) signal. When a PC powers on, the power supply performs self-tests. Once internal voltages (+3.3V, +5V, +12V) stabilize, the PSU sends a +3V to +5V signal to the motherboard on the PWR_OK pin. The motherboard timer circuit receives this signal and releases the CPU reset line, allowing the CPU to begin executing BIOS/UEFI code. If PWR_OK remains at 0 VDC, the CPU is permanently held in reset, causing fans to spin without POST, beep codes, or chipset control.
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Power Good (PWR_OK) Signal and CPU Reset Mechanism
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