Examkin

Question

Difficulty: MediumMotherboard, RAM, CPU, and Power Issues

A technician is troubleshooting a desktop workstation that suddenly powered off during normal operation and now fails to boot. When the power button is pressed, the system power LEDs illuminate and cooling fans spin continuously, but the computer displays no video output and produces no diagnostic POST beep codes. Testing the main 24-pin ATX power connector with a multimeter reveals that the +3.3 V+3.3\text{ V}, +5 V+5\text{ V}, and +12 V+12\text{ V} voltage rails are all operating within normal tolerances, but the Power_Good (PWR_OK) signal line reads 0 V0\text{ V}. Which of the following is the most likely cause of this failure?

  1. The power supply is failing to signal that voltages are stable, holding the motherboard and CPU in a continuous reset state.Answer
  2. B
    The graphics processing unit has suffered a total hardware failure and is preventing the video BIOS from initializing.
  3. C
    The memory modules are installed in non-matching motherboard slots, preventing dual-channel communication during POST.
  4. D
    The processor thermal paste has completely degraded, forcing the CPU to remain in a permanent hardware power-off lock.

Answer

The power supply is failing to signal that voltages are stable, holding the motherboard and CPU in a continuous reset state.
The correct answer accurately identifies the role of the Power_Good (PWR_OK) signal. When a power supply powers on, its internal logic verifies that the +3.3 V+3.3\text{ V}, +5 V+5\text{ V}, and +12 V+12\text{ V} outputs have stabilized. Once stable, it sends a +5 V+5\text{ V} signal down the PWR_OK wire (Pin 8 of the 24-pin ATX connector) to the motherboard. If this signal is 0 V0\text{ V} or missing, the motherboard's clock generator keeps the CPU in a continuous reset state, preventing the system from initiating POST or displaying video even though power fans and status LEDs are receiving basic electricity.

Step-by-Step Solution

1
Analyze the observed system symptoms and multimeter measurements.
The main voltage rails (+3.3 V+3.3\text{ V}, +5 V+5\text{ V}, +12 V+12\text{ V}) are within spec, but the PWR_OK signal line measures 0 V0\text{ V} while fans and LEDs are receiving basic power.
Power supply units output a +5 V+5\text{ V} Power_Good signal (PWR_OK, pin 8 on the 24-pin ATX connector) to the motherboard after internal circuitry confirms all power rails are stable and clean.
2
Determine the impact of a missing PWR_OK signal on motherboard operation.
Without a logic HIGH (+3 V+3\text{ V} to +5 V+5\text{ V}) PWR_OK signal, the motherboard timer chip holds the CPU reset line active.
Holding the CPU in continuous reset prevents damage from unstable power and stops the CPU from executing instructions, meaning POST cannot start, no video is rendered, and no POST error codes are generated.
3
Identify the proper corrective action.
Replace the failing power supply unit.
Since the PSU is failing to output the required PWR_OK signal despite providing voltage on main rails, the internal power monitoring circuit of the PSU is defective.

Key Concept

Power Supply Power_Good (PWR_OK) Signal Functionality during POST
Rate this question

Topics

Motherboard, RAM, CPU, and Power Issues

More questions from this topic