Diagnosing a Failing Power Supply Under Load
A technician is troubleshooting a desktop computer that randomly freezes and then reboots. The technician runs a CPU stress test; after about 5 minutes, the system freezes and reboots. The CPU temperature during the test is monitored and never exceeds 75°C. The technician has run a full memory test (MemTest86) and a hard drive diagnostic (SMART check); both pass without errors. The power supply voltages are within specifications when measured with a multimeter at idle and under load. Which of the following is the MOST likely cause of the issue?
Quick Answer
This question rewards recognizing that a power supply can pass every static test and still fail dynamically. The technician ruled out heat (CPU stayed under 75°C), memory (MemTest86 passed), and storage (SMART check passed), and even measured voltages with a multimeter at idle and under load, yet the system still froze and rebooted during sustained stress. A multimeter only captures a snapshot in time; it cannot detect the brief voltage ripple or slow transient response that occurs when a marginal PSU struggles to keep up with rapidly changing current demand during a real workload. Once every other component has been cleared through legitimate diagnostics, an intermittent failure that appears specifically under sustained load is the classic signature of a power delivery problem that basic voltage checks cannot catch. The lesson to carry forward is that passing a static voltage test does not clear the PSU entirely; when a system is stable at idle and under light use but fails specifically after several minutes of stress testing, and every other subsystem has already tested clean, look past the numbers on the multimeter and treat the power supply as the most likely remaining suspect, since instability under dynamic load is often invisible to simple measurement tools.
⚠ Common exam trap
CompTIA often tests the misconception that a multimeter reading of correct voltages rules out PSU failure, but the trap is that intermittent PSU faults under sustained load—especially ripple or transient dropout—are invisible to a static voltage measurement.
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
✓
A failing power supply unit
The symptoms—random freezing and rebooting under CPU stress, with all other components (CPU temperature, memory, storage, and power supply voltages) passing diagnostics—point to a failing power supply unit (PSU). Even though the multimeter shows correct voltages at idle and under load, a PSU can fail intermittently under sustained high current draw, causing voltage ripple or transient drops that trigger a system reset. The CPU stress test specifically taxes the 12V rail, and a marginal PSU may not maintain stable output during extended high-load scenarios, leading to the observed behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Insufficient voltage to the CPU due to motherboard VRM failure
Why it's wrong here
While a VRM failure could cause instability, the CPU stress test would likely show voltage droops immediately, and the voltages measured at the PSU would not directly show VRM output. But more commonly, a failing PSU causes such symptoms even if DC voltages appear normal.
When this WOULD be correct
A technician measures power supply voltages at idle only and finds them normal, but under CPU load, the system immediately crashes or fails to boot. The multimeter shows voltage drops on CPU power rails during load, indicating VRM failure.
- ✓
A failing power supply unit
Why this is correct
A power supply can produce correct DC voltages but still have excessive ripple or poor transient response under dynamic load changes, causing the system to freeze or reboot. This is a common intermittent failure that voltage checks alone may not catch.
- ✗
A corrupted operating system driver
Why it's wrong here
A corrupted driver could cause crashes, but the system freezes during a CPU stress test (which stresses hardware, not drivers) suggests a hardware issue. Moreover, a driver problem would more likely cause a blue screen (BSOD) rather than a freeze and reboot.
When this WOULD be correct
This option would be correct if the system exhibited blue screens with driver-specific error codes (e.g., IRQL_NOT_LESS_OR_EQUAL) or if the issue occurred only when a particular device driver was loaded, and hardware diagnostics (RAM, CPU, PSU) all passed.
- ✗
A failing system fan that spins intermittently
Why it's wrong here
If a system fan fails, temperatures would rise, but the CPU temperature remained within safe limits (75°C). While other components could overheat, the CPU stress test primarily stresses the CPU, and its temperature was fine.
When this WOULD be correct
A desktop computer randomly shuts down after a few minutes of use, and the CPU temperature quickly rises to 95°C. The technician notices the CPU fan stops spinning intermittently. In this scenario, a failing system fan is the most likely cause.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 220-1101 exam frequently reuses these exact scenarios with slightly different constraints.
✓A failing power supply unitCorrect answer▾
Why this is correct
A power supply can produce correct DC voltages but still have excessive ripple or poor transient response under dynamic load changes, causing the system to freeze or reboot. This is a common intermittent failure that voltage checks alone may not catch.
✗Insufficient voltage to the CPU due to motherboard VRM failureWrong answer — click to see why▾
Why this is wrong here
The power supply voltages are measured within specifications at idle and under load, which includes the CPU voltage rails. A VRM failure would typically cause voltage fluctuations or droop under load, but the multimeter readings show no such issue, and the CPU stress test runs for 5 minutes before freezing, not immediately.
★ When this WOULD be the correct answer
A technician measures power supply voltages at idle only and finds them normal, but under CPU load, the system immediately crashes or fails to boot. The multimeter shows voltage drops on CPU power rails during load, indicating VRM failure.
Why candidates choose this
Candidates may confuse VRM failure with PSU failure because both can cause instability under load, and they might assume that if PSU voltages are okay, the VRM must be the culprit, overlooking that PSU issues can be intermittent or not captured by static voltage measurements.
✗A corrupted operating system driverWrong answer — click to see why▾
Why this is wrong here
A corrupted OS driver would typically cause blue screens or errors with specific error codes, not a consistent freeze after 5 minutes of CPU stress testing. The hardware diagnostics (MemTest86, SMART, PSU voltages) all passed, pointing to a hardware issue under sustained load, not software.
★ When this WOULD be the correct answer
This option would be correct if the system exhibited blue screens with driver-specific error codes (e.g., IRQL_NOT_LESS_OR_EQUAL) or if the issue occurred only when a particular device driver was loaded, and hardware diagnostics (RAM, CPU, PSU) all passed.
Why candidates choose this
Candidates may assume that random freezes and reboots are often caused by software corruption, especially when hardware tests initially pass. They overlook that stress testing specifically triggers the issue, indicating a hardware problem under load.
✗A failing system fan that spins intermittentlyWrong answer — click to see why▾
Why this is wrong here
The system freezes under CPU stress but CPU temperature remains below 75°C, and the fan is not mentioned as failing during monitoring; a failing system fan that spins intermittently would likely cause overheating and thermal throttling or shutdown, but the temperature is normal, so this is not the cause.
★ When this WOULD be the correct answer
A desktop computer randomly shuts down after a few minutes of use, and the CPU temperature quickly rises to 95°C. The technician notices the CPU fan stops spinning intermittently. In this scenario, a failing system fan is the most likely cause.
Why candidates choose this
Candidates may associate random freezes and reboots with cooling issues, and a failing fan is a common hardware problem, but they overlook that the temperature is within normal limits, so the fan is likely working correctly.
Analysis generated from the official 220-1101blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Same concept, more angles
1 more way this is tested on 220-1101
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A technician is troubleshooting a desktop computer that experiences intermittent freezing, especially when running graphics-intensive applications or games. The technician has updated all drivers, run full antivirus scans, and replaced the RAM. The CPU and GPU temperatures stay below 70°C under load. The event logs show no critical errors. However, the freezing always occurs under high load, and sometimes the system restarts without a blue screen. Which component is MOST likely failing?
hard- ✓ A.Power supply unit (PSU)
- B.CPU
- C.GPU
- D.Motherboard
Why A: The power supply unit (PSU) is the most likely failing component because intermittent freezing and restarts under high load, without blue screens or critical errors, are classic symptoms of an unstable or insufficient power supply. When the GPU and CPU draw maximum current during graphics-intensive tasks, a failing PSU may drop below required voltage rails (e.g., +12V), causing the system to lock up or reboot. The fact that temperatures are normal and RAM has been replaced eliminates thermal and memory issues, pointing directly to power delivery.
JA
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.