220-1101 Hardware and Network Troubleshooting Practice Question
A desktop computer will not power on. The technician presses the power button, but there are no signs of activity—no fans, no LEDs. The power supply unit (PSU) is securely connected and the wall outlet is working. The technician uses a multimeter to check the PSU's main power connector. The technician finds that the green wire (PS_ON) goes low (0V) when the power button is pressed, indicating the motherboard is sending the signal to turn on. However, no other pins on the main connector show any voltage, and the PSU fan does not spin. Which of the following is the most likely cause?
⚠ Common exam trap
Many exam-takers assume the motherboard is at fault because the system is dead, but the PS_ON signal being low proves the motherboard is doing its job, shifting the fault to the PSU's internal power conversion.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The power supply unit has failed internally.
The green wire (PS_ON) going low (0V) confirms the motherboard is correctly signaling the PSU to turn on. Since no other pins show voltage and the PSU fan does not spin, the PSU is failing to convert AC to DC and distribute power internally, indicating an internal failure. A working PSU would respond to the PS_ON signal by activating its fan and outputting voltages on the other pins.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The power button is faulty.
Why it's wrong here
The power button is a momentary switch that pulls PS_ON to ground (low) when pressed. If it were faulty, the signal would not change state, but the technician observes the signal going low, confirming the button and front-panel header/wiring are electrically sound. Therefore the button is not the cause.
- ✗
The motherboard is not providing power to the PSU.
Why it's wrong here
The motherboard does not send power to the PSU; rather, it asserts a low-true logic signal called PS_ON on pin 16 of the 24-pin ATX connector. The PSU itself receives mains AC and standby power independently, and the motherboard's only role is to command the PSU to turn on. Because the signal is being received, the motherboard's power-on control circuitry is functional.
- ✓
The power supply unit has failed internally.
Why this is correct
The correct answer is that the PSU has failed internally. Even with a valid PS_ON signal, the PSU's internal switching regulators or output stages are not producing the required +3.3V, +5V, and +12V rails. A failed PSU must be replaced, as internal components are not user-serviceable and the failure can also indicate potential damage to downstream components.
- ✗
The CPU is not seated correctly.
Why it's wrong here
If the CPU were loose or not seated, the motherboard's CPU power delivery circuitry would likely fail to initialize, but the PSU would still receive the PS_ON signal and turn on, producing all main voltages before the motherboard's diagnostic LEDs or POST code display issues. The symptom here is a complete lack of power output, which is consistent with a PSU failure, not a CPU seating problem.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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