- A
The power supply is faulty and needs replacement.
The 5VSB is present and PS_ON goes low when the button is pressed, showing the motherboard is sending the correct signal. The PSU fails to start, indicating an internal fault. Replacement is the appropriate action.
- B
The motherboard is not sending the PS_ON signal, likely due to a fault.
Why wrong: The PS_ON signal being low (0V) when the button is pressed is normal and expected. The motherboard is correctly pulling this line low to command the PSU to power on. Thus, the motherboard is not faulty.
- C
The power supply's +12V rail is shorted, causing overcurrent protection.
Why wrong: A shorted +12V rail would likely trigger overcurrent protection, which might cause the PSU to shut down entirely, including 5VSB. Since 5VSB is stable at 4.9V, a short is unlikely.
- D
The front panel power switch is stuck in the closed position.
Why wrong: A stuck front-panel power switch would keep PS_ON low continuously, not just when pressed. The measurement shows PS_ON goes low only when the button is pressed, so the switch is functioning normally.
220-1201 PS_ON signal Practice Question
This 220-1201 practice question tests your understanding of power supply. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: pS_ON signal. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A desktop PC fails to power on. The technician measures the voltage on the 24-pin motherboard connector and finds that the purple wire (5VSB) reads 4.9V, but the green wire (PS_ON) reads 0V when the power button is pressed. What does this indicate?
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 is faulty and needs replacement.
A desktop PC fails to power on. Measurements: 5VSB is present (4.9V is within tolerance), indicating the power supply is receiving AC power and its standby circuitry is working. When the power button is pressed, PS_ON is pulled low (0V) by the motherboard, which is the correct signal to request the PSU to turn on. Since the PSU does not start (fans don't spin, no other outputs), the fault lies within the power supply itself. A faulty PSU may have a failed main switching circuit or other internal defect preventing it from responding to the PS_ON signal. Therefore, the power supply should be replaced. Option B is incorrect because a low PS_ON indicates the motherboard is functioning correctly in this regard.
Key principle: PS_ON signal
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 supply is faulty and needs replacement.
Why this is correct
The 5VSB is present and PS_ON goes low when the button is pressed, showing the motherboard is sending the correct signal. The PSU fails to start, indicating an internal fault. Replacement is the appropriate action.
Related concept
PS_ON signal
- ✗
The motherboard is not sending the PS_ON signal, likely due to a fault.
Why it's wrong here
The PS_ON signal being low (0V) when the button is pressed is normal and expected. The motherboard is correctly pulling this line low to command the PSU to power on. Thus, the motherboard is not faulty.
- ✗
The power supply's +12V rail is shorted, causing overcurrent protection.
Why it's wrong here
A shorted +12V rail would likely trigger overcurrent protection, which might cause the PSU to shut down entirely, including 5VSB. Since 5VSB is stable at 4.9V, a short is unlikely.
- ✗
The front panel power switch is stuck in the closed position.
Why it's wrong here
A stuck front-panel power switch would keep PS_ON low continuously, not just when pressed. The measurement shows PS_ON goes low only when the button is pressed, so the switch is functioning normally.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
The PS_ON signal being low (0V) when the button is pressed is normal and expected. The motherboard is correctly pulling this line low to command the PSU to power on. Thus, the motherboard is not faulty.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- PS_ON signal
- 5VSB (standby voltage)
- Power supply fault
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
PS_ON signal
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. PS_ON signal Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Review pS_ON signal, then practise related 220-1201 questions on the same topic to reinforce the concept.
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Power Supply — study guide chapter
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Power Supply practice questions
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Power Supply — This question tests Power Supply — PS_ON signal.
What is the correct answer to this question?
The correct answer is: The power supply is faulty and needs replacement. — A desktop PC fails to power on. Measurements: 5VSB is present (4.9V is within tolerance), indicating the power supply is receiving AC power and its standby circuitry is working. When the power button is pressed, PS_ON is pulled low (0V) by the motherboard, which is the correct signal to request the PSU to turn on. Since the PSU does not start (fans don't spin, no other outputs), the fault lies within the power supply itself. A faulty PSU may have a failed main switching circuit or other internal defect preventing it from responding to the PS_ON signal. Therefore, the power supply should be replaced. Option B is incorrect because a low PS_ON indicates the motherboard is functioning correctly in this regard.
What should I do if I get this 220-1201 question wrong?
Review pS_ON signal, then practise related 220-1201 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
PS_ON signal
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Last reviewed: Jun 18, 2026
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