Fix 0x801F0003 ERROR_FLT_INVALID_ASYNCHRONOUS_REQUEST
This error means a filter manager driver tried an async operation where sync is required. Usually a bad driver or a corrupt filter stack. Here's how to pin it down.
Quick answer for advanced users
Open an elevated command prompt and run fltmc instances to list loaded minifilter drivers. Identify the one with a high number of attached instances that matches the timing of your issue. Then use fltmc unload <driver_name> to remove it temporarily. If the error stops, that driver is the cause. Update or remove it permanently.
What this error actually means
I know this error is infuriating — it pops up out of nowhere, often during file operations, and there's nothing in the Event Viewer that screams "fix me." I ran into this on a Windows Server 2019 box running backup software. The error 0x801F0003, ERROR_FLT_INVALID_ASYNCHRONOUS_REQUEST, is thrown by the filter manager (fltmgr.sys) when a minifilter driver tries to complete an I/O request as asynchronous, but the operation type (like a synchronous create or close) explicitly requires synchronous completion. Why? Usually because the driver is buggy, outdated, or the filter stack got corrupted after an update. You'll see this most often with antivirus, backup agents, or storage drivers — they hook into the file system and sometimes mess up the async flag.
Step-by-step fix
Step 1: Identify the filter driver causing the issue
Open Command Prompt as Administrator. Run:
fltmc instancesThis dumps every loaded minifilter driver, its altitude (load order), and instance count. Look for drivers you didn't install — especially third-party ones. Write down the names that look suspicious. On my server, it was Symantec Eraser Control Driver (that backup agent).
Step 2: Unload the suspect driver temporarily
If you have a guess, unload it and test. Run:
fltmc unload <DriverName>Replace <DriverName> with the exact name from step 1. No quotes. If the error disappears, you've found it.
Step 3: Check which driver is failing
If you're not sure which driver is the culprit, hook up a debugger or use fltmc to monitor. But the simpler way: look at your installed programs list. Anything related to backup, antivirus, or encryption? Uninstall it temporarily. Reboot, and see if the error returns.
Step 4: Update or remove the offending driver
Once identified, update the driver from the vendor's website — not Windows Update. If no update exists, uninstall the software or use the vendor's custom removal tool. Some drivers (like old Symantec or McAfee ones) leave leftovers. Run fltmc instances again after removal to confirm it's gone.
Alternative fixes when the main one fails
Fix 1: Use Driver Verifier to pinpoint
If no clear driver stands out, enable Driver Verifier for non-Microsoft drivers. Be careful — this can cause crashes. Open Verifier (verifier.exe), select "Create custom settings," then "Select individual settings," check everything except Low Resources Simulation. Next, "Select driver names from a list," then click "Provider" to sort, and select only third-party drivers. Reboot. If a crash dump points to a driver, that's your culprit. Then run verifier /reset to turn it off and remove that driver.
Fix 2: Repair the filter stack
Corruption in the filter stack can cause this. Open Command Prompt as Admin and run:
sfc /scannowThen repair Windows image:
DISM /Online /Cleanup-Image /RestoreHealthReboot. If the error persists, the stack itself is fine — it's a driver issue.
Fix 3: Boot into Safe Mode
Safe Mode loads minimal drivers. If the error doesn't appear there, a third-party driver is the problem. Boot into Safe Mode (Shift + Restart from login screen, then Troubleshoot > Startup Settings). If the error is gone, boot normally and methodically uninstall suspect software.
Prevention tip
Never install software that hooks into the file system without checking for filter driver updates first. Antivirus, backup tools, and disk encryption are the biggest offenders. Keep them updated. Before applying major Windows updates, temporarily unload third-party filter drivers using fltmc unload — I've had a Windows 10 22H2 update break a backup driver this way. Reboot, then reinstall the driver after the update. And always check fltmc instances after installing new software — a bloated filter stack is a ticking time bomb.
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