hardMultiple ChoiceObjective-mapped
220-1201 PS_ON signal Practice Question
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.
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.
- ✗
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 short circuit on a major power rail, such as the +12V rail, would typically activate the power supply's internal Over Current Protection (OCP) or Short Circuit Protection (SCP) mechanisms. These safety features are designed to shut down the power supply completely, often including the 5VSB (5V Standby) rail, to prevent damage. Since the 5VSB is stable at 4.9V, it indicates that the standby circuitry is operational and not being shut down by a catastrophic short on one of the main output rails.
- ✗
The front panel power switch is stuck in the closed position.
Why it's wrong here
If the front panel power switch were stuck in the closed (on) position, the PS_ON signal from the motherboard to the power supply would be continuously held at a low voltage (0V) as soon as the PC was plugged in, without any interaction. The observation that PS_ON only goes low when the power button is actively pressed indicates that the switch is functioning correctly, making contact only when actuated, and is not the source of the power failure.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1201 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1201 exam.