A failing power supply is one of the trickiest components to diagnose because its symptoms mimic so many other problems — random reboots, blue screens under load, drives that intermittently disappear, or a system that simply won't power on. Before you spend money on a replacement PSU or, worse, misdiagnose the fault as a motherboard or RAM problem, a handful of free software tools can help you check voltage stability and spot warning signs from within Windows. This guide walks through the best of them and explains how to read the numbers they give you.

Understanding What You're Measuring

A PSU converts mains AC into several DC voltage rails — primarily +12V (which powers the CPU, GPU, and most of the system's demanding components), +5V, and +3.3V. ATX specification tolerances allow these rails to vary by roughly ±5%, meaning a nominal +12V rail should sit somewhere between about 11.4V and 12.6V under normal conditions. Voltages that sag significantly under load, or that fluctuate erratically at idle, are the clearest software-visible sign of a struggling PSU.

It's worth being upfront about a limitation: software monitoring reads voltages reported by the motherboard's sensors, which are convenient but not laboratory-grade accurate. They're excellent for spotting trends and obvious problems, less reliable for precise voltage verification — for that, a hardware PSU tester or multimeter is more accurate. Still, software is the right first step because it's free, immediate, and can be checked under real load.

HWMonitor

HWMonitor by CPUID is one of the most widely used free monitoring tools and a sensible starting point for PSU checks.

  • What it shows: live readings for +12V, +5V, and +3.3V rails, alongside CPU/GPU temperatures and fan speeds, all in one simple window.
  • How to use it: open HWMonitor, locate your motherboard entry in the list, and expand it to find the voltage rails. Note the "Min", "Value", and "Max" columns — these show the lowest and highest readings recorded since you opened the app, which is exactly what you want for spotting instability.
  • Interpreting results: run a demanding task (a game, a benchmark, or a stress test) while HWMonitor is open in the background, then check the Min/Max spread afterwards. A rail that dips well outside the ±5% tolerance under load, or swings wildly even at idle, points towards a PSU that's struggling to maintain stable output.

Pro tip: Run HWMonitor during a stress test rather than at idle. A PSU can look perfectly healthy with the system doing nothing, then sag noticeably the moment the CPU and GPU are both pulling significant power simultaneously — which is exactly the scenario that triggers real-world crashes.

HWiNFO

HWiNFO goes further than HWMonitor for anyone comfortable with a more detailed interface, and it's a favourite among enthusiasts for exactly this reason.

  • What it adds: a "Sensors" view with far more granular voltage rail data on many motherboards, plus a built-in logging feature that records readings to a file over time — useful if a shutdown happens sporadically and you want to capture what the voltages were doing in the seconds before it occurred.
  • How to use it: launch HWiNFO in "Sensors-only" mode, open the Sensors window, and locate the voltage section for your motherboard. Enable logging (File menu) before a gaming session or workload you know tends to trigger the fault, then review the log afterwards for voltage dips that correlate with the crash time.
  • Why it's useful for intermittent faults: unlike watching a live window, logging lets you walk away and review data after the fact — essential for PSU faults that only show up occasionally rather than on demand.

MSI Afterburner (with RTSS On-Screen Display)

Though best known as a GPU overclocking tool, MSI Afterburner (works with any GPU brand, not just MSI) paired with its RivaTuner Statistics Server overlay lets you watch voltage and power draw live, overlaid directly on games — useful for catching a crash the exact moment it happens rather than checking a separate window afterwards.

  • Best for: gamers experiencing crashes specifically during gaming sessions, where GPU power draw spikes are often the trigger for a marginal PSU to fail.
  • How to use it: enable the on-screen display in Afterburner's settings, add GPU power draw and 12V rail readings to the overlay, and watch for correlation between power spikes and instability.

Windows Event Viewer (Free, Built In)

Windows itself logs unexpected shutdowns and power-related errors, and it's worth checking alongside the monitoring tools above rather than instead of them.

  • Where to look: open Event Viewer, navigate to Windows Logs > System, and filter for Critical and Error level events around the time of a crash.
  • What to look for: a "Kernel-Power" event with Event ID 41 typically indicates the system lost power or restarted unexpectedly without a clean shutdown — a classic PSU-related signature, though it can also point to other causes, so treat it as a corroborating clue rather than definitive proof.

Pro tip: Cross-reference the timestamp of a Kernel-Power 41 event in Event Viewer with any HWiNFO voltage log you have running. If a voltage dip lines up precisely with the crash timestamp, that's strong evidence pointing at the PSU specifically.

When Software Isn't Enough

If software monitoring shows borderline or inconclusive results, the next step up is a dedicated hardware PSU tester — a small standalone device that plugs directly into the PSU's connectors and reports rail voltages without needing the rest of the system running, isolating the PSU from every other possible cause. These are inexpensive and widely available, and they're the right next step when software readings are ambiguous or when the PSU won't even power the system on for software to run in the first place.

Putting It Together

A sensible diagnostic sequence is: check Event Viewer for Kernel-Power 41 events first, run HWMonitor or HWiNFO during a real workload to look for voltage instability under load, and if anything looks borderline, confirm with a dedicated PSU tester before spending money on a replacement. This combination of free software and one inexpensive hardware tool will correctly diagnose the large majority of PSU-related instability without guesswork.

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