0x00000021 Fixes for QUOTA_UNDERFLOW on Windows
Stop code 0x00000021 can appear after a recent driver, software, or hardware change. A practical first response is to undo the newest change, then install pending Windows updates and retest. If the blue screen continues, use the sections below to narrow it down by when the crash happens and what changed just before it started.
This guide is part of our Windows BSOD stop codes: full list and fixes series.
Fast triage in 3 steps: first, undo the newest driver, utility, or device change; second, check whether the crash happens at startup, shutdown, or under load; third, if the pattern survives rollback and uninstall work, move to Windows repair commands and memory testing.
| If the crash happens… | Start here | What to check first |
|---|---|---|
| During startup | Safe Mode, then remove or roll back recent storage, network, GPU, or chipset drivers | Boot-critical driver or recent device stack change |
| During shutdown | Clean boot, then uninstall the newest antivirus, VPN, backup, or encryption tool | Third-party service or driver that loads during shutdown |
| During gaming or heavy load | Revert recent GPU-related changes, then run memory diagnostics | GPU driver path or memory-related instability |
For broader context, see this BSOD troubleshooting hub and this driver troubleshooting hub.
What does stop code 0x00000021 mean?
Officially, QUOTA_UNDERFLOW has a bug check value of 0x00000021, and it indicates that quota charges have been mishandled by returning more quota to a particular block than was previously charged. (source)
Why this stop code usually points to kernel-mode software
Because the bug check refers to quota handling inside Windows, recent low-level changes deserve attention first. That includes newly installed or recently updated drivers, device-related software, and utilities that integrate closely with storage, networking, or security.
When the timing is clear, start with the newest change rather than making multiple changes at once.
Likely causes, ranked by odds
Use the first crash date and any recent system changes to decide where to start. Device drivers, newly added hardware, and recently installed software are the most direct items to review because they can be reversed with standard Windows tools.
- Recent driver changes. Check recent storage, chipset, network, and GPU driver changes in Start > Device Manager.
- Recently installed low-level software. If the BSOD began after installing security software, backup software, VPN software, encryption software, or virtualization software, test with a clean boot and remove the newest item if the system stabilizes.
- Windows file corruption or hardware checks. If no recent change lines up with the first crash, open an elevated Command Prompt and run
sfc /scannow; if it finds corruption or the BSOD still returns, runDISM /Online /Cleanup-Image /RestoreHealth; after that, schedule a memory test with mdsched.exe.
According to Bug Check 0x21 QUOTA_UNDERFLOW – Windows drivers — The bug check value for QUOTA_UNDERFLOW is 0x00000021. (source)
Did 0x00000021 start after a driver or device change?
If the first crash started right after a driver install, firmware utility, dock, GPU package, storage update, or newly attached device, reverse that change first. Device Manager and a simple disconnect test are the fastest standard checks.
Roll back or remove the newest driver
If the first crash followed a driver install or device-related software update, treat that change as the starting point until testing proves otherwise.
- Open Start > Device Manager.
- Expand Storage controllers, IDE ATA/ATAPI controllers, Network adapters, Display adapters, and System devices.
- Right-click the recently changed device > Properties > Driver.
- Select Roll Back Driver if it is available. If it is not, choose Uninstall Device, restart, and let Windows load the prior or inbox driver.
- If the crash began after new hardware, shut down the PC, disconnect only that newly added device, start Windows again, and use the PC long enough to repeat the normal trigger before reconnecting it.
Apply updates in the right order
Windows Update is worth doing early, especially after a rollback or uninstall, so Windows can install any pending fixes.
