Faulty or unstable memory is one of the hardest PC problems to pin down. NZXT's support guide lists the typical symptoms: games and applications freezing or crashing, random restarts, blue screens, no signal to the monitor and a PC that will not finish booting into Windows. Because those symptoms overlap with drivers, power and overheating problems, testing the RAM properly is the quickest way to rule it in or out.
Forge PC has not tested these tools on our own hardware for this guide. The steps come from Microsoft's documentation, NZXT's support centre and the official MemTest86 and Memtest86+ websites.
Tool | Runs from | Cost | Best for |
|---|
Windows Memory Diagnostic (mdsched) | Windows or the Windows Recovery Environment | Built in | A quick first check |
MemTest86 (PassMark) | Bootable USB drive | Free edition; paid Pro and Site editions | A thorough test, including when Windows will not load |
Memtest86+ | Bootable USB drive | Free, open source (GPL v2) | A thorough test with an open-source tool |
OCCT memory test | Inside Windows | Free download | Testing stability while Windows is running |
Memtest86+ and MemTest86 are different programs, despite the names. The Memtest86+ site points out that it "is not an edition of Memtest86", which has been closed-source "Freemium" software owned by PassMark since 2013.
Before you start: undo memory overclocks
If you have enabled an XMP or EXPO profile, or tuned your memory manually, that is an overclock. The Memtest86+ FAQ's first piece of advice when errors appear is: "If your computer is overclocked, you should try again at default settings first." It is worth testing at default settings and at your XMP/EXPO profile separately, so you know whether the memory is faulty or the profile is simply unstable on your system. Our XMP and EXPO guide explains how to switch the profile on and off.
Option 1: Windows Memory Diagnostic
This is built into Windows and needs no download.
Search the Start menu for Windows Memory Diagnostic, or press Windows + R and type mdsched.exe.
Choose whether to restart and test now, or to test at the next restart.
The test runs automatically after the restart. Microsoft's instructions say you can press F1 to change the Test Mix to Basic, Standard or Extended, then F10 to apply. Extended is slower but more thorough.
When it finishes, the PC restarts on its own.
To read the result, NZXT's guide says to open Event Viewer, go to Windows Logs > System, and look for an Information entry with the source MemoryDiagnostics-Results.
If you cannot get into Windows, NZXT says you can run mdsched from the Command Prompt in the Windows Recovery Environment (Troubleshoot > Advanced options) or from a Windows installation USB, although the results may not be saved in that case.
Option 2: MemTest86 from a USB drive
MemTest86 boots from a USB flash drive and tests the RAM without loading Windows. PassMark describes it as a free stand-alone memory tester with 14 test algorithms, support for DDR4 and DDR5, and a Secure Boot-signed boot image.
Use a spare USB stick. Creating the boot drive wipes everything on it.
Download the free edition from PassMark's MemTest86 website and extract the .zip file.
Run the included imageUSB program. NZXT's steps are: select the USB drive, choose Write image to USB Drive with Fast Image Verification enabled, browse to the .img file if needed, and click Write.
Plug the drive into the PC you want to test, open your motherboard's boot menu at start-up (or the boot override option in the BIOS) and select the USB drive.
MemTest86 starts automatically after a few seconds. NZXT notes that it saves a report to the flash drive when the test finishes, and can identify errors on individual modules.
Option 3: Memtest86+ from a USB drive
Memtest86+ is the free, open-source alternative. Version 9.00, released on 15 October 2026, added a bus stress test that its developers say targets DDR5 interface problems, including "XMP/EXPO training issues", along with a function to save a report to a USB drive by pressing F6.
Download the Windows USB Installer from memtest.org.
Plug in a standard FAT32-formatted USB drive, as the site instructs (use one you do not mind wiping), then run the installer and follow the steps.
Boot the PC from the USB drive using your motherboard's boot menu. The site says this is usually F8, F12 or Esc at start-up, but check your manual.
Two things to know. First, if you see a Secure Boot violation, the FAQ explains that Memtest86+ 9.00 is signed but "not by Microsoft yet": you can temporarily disable Secure Boot in the BIOS, or add its certificate to your firmware's database. Remember to turn Secure Boot back on afterwards. Second, Memtest86+ never stops on its own. The FAQ says it runs in an endless loop: once one pass completes successfully, all of your memory has been tested, and you can press Esc to stop or leave it longer for more confidence.
Option 4: OCCT inside Windows
If Windows boots normally, NZXT also suggests the free OCCT stability tool. Select its Memory test and run it with the default settings; any errors are logged in the program.
What to do if the test finds errors
Go back to default memory settings and test again. If the errors disappear, the XMP/EXPO profile or a manual overclock is the problem, not the memory itself.
Test one module at a time. The Memtest86+ FAQ recommends removing modules one at a time to find the faulty one, or rotating them between slots.
Check the basics: modules fully seated, in the slots your motherboard manual recommends, and a BIOS that is up to date. Our BIOS update guide covers that safely.
Replace faulty memory under warranty. NZXT advises contacting the system builder if it is a pre-built PC under warranty, or the RAM manufacturer otherwise.
If the memory passes but the PC still crashes, work through our blue screen guide and PC won't boot guide for the other usual causes.
Our verdict
Start with Windows Memory Diagnostic for a quick check, but use a bootable MemTest86 or Memtest86+ USB drive for a thorough test, especially if Windows will not load. Test at default memory settings first, because an unstable XMP or EXPO profile can look exactly like faulty RAM. If errors persist at stock settings, test one module at a time and claim a replacement under warranty. We have not tested these tools ourselves; the steps come from Microsoft, NZXT and the MemTest86 and Memtest86+ developers.
