220-1101 Hardware Practice Question
A technician is troubleshooting a desktop computer that does not power on. The motherboard's power LED is lit. When the power button is pressed, no fans spin and no beeps occur. The technician uses a multimeter to test the power supply. The purple wire (+5VSB) reads 5.1V. The green wire (PS_ON) reads 0V when the power button is pressed. However, all other pins on the main power connector show 0V. Which of the following is the most likely cause?
⚠ Common exam trap
The trap here is that candidates see the motherboard power LED lit and the PS_ON signal working and incorrectly assume the PSU is good, when in fact the PSU's main output stage is completely dead despite receiving the correct enable signal.
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 is faulty.
The +5VSB (purple wire) reading of 5.1V confirms the standby power rail is working, and the PS_ON (green wire) dropping to 0V when the power button is pressed indicates the motherboard is correctly asserting the power-on signal. However, all other pins showing 0V means the power supply is not turning on its main outputs (+12V, +5V, +3.3V). This is a classic symptom of a faulty power supply unit (PSU) that fails to energize the main rails despite receiving a valid 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 motherboard is faulty.
Why it's wrong here
The motherboard receives standby power and responds to the power button by pulling the PS_ON line low, which is the correct turn-on command. If the motherboard were faulty, it would likely fail to assert PS_ON, leaving the PSU in standby mode with no LED reaction or with the power button unresponsive. Since the LED lights and the signal transitions are present, the motherboard's power management circuitry is functioning; a failed motherboard would not produce this specific behavior.
- ✗
The power button is defective.
Why it's wrong here
The power button's operation is directly verified by the fact that pressing it causes the PS_ON line to drop from 5V to ground, which is exactly what the motherboard requires to signal the PSU. A defective button—whether due to a broken switch, loose connector, or shorted trace—would leave PS_ON high, and the PSU would never receive the turn-on signal. The observed 0V reading confirms the button and its wiring to the front-panel header are electrically sound.
- ✓
The power supply unit is faulty.
Why this is correct
The power supply is the only component that interprets the PS_ON signal and is responsible for generating the +3.3V, +5V, and +12V rails after a short delay. With PS_ON low, the PSU should start; its failure to produce any main voltage indicates an internal fault such as a blown switching transistor, failed PWM controller, or degraded output capacitors. The standby rail remains active because it is derived from a separate circuit, allowing the LED and PS_ON signal to work normally even though the main converter is dead.
- ✗
The CPU is not seated properly.
Why it's wrong here
A CPU that is not seated properly does not affect the ATX power sequencing; the PSU will still receive PS_ON and produce all main voltages, and the fans will spin. The problem appears only later during the POST process, when the CPU cannot communicate with the chipset, leading to a blank screen or beep codes. Here, the absence of main DC voltages points to a failure before the CPU ever becomes relevant, ruling out a socket or seating issue as the cause.
Go deeper
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Key term
Power supply unit
A power supply unit (PSU) is the hardware component that converts electricity from a wall outlet into usable power for a computer's internal components.
Key term
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A light-emitting diode (LED) is a semiconductor device that emits light when an electric current passes through it, commonly used as an indicator or illumination source in electronic devices.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 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 220-1101 exam.