Fix ERROR_FLT_INSTANCE_ALTITUDE_COLLISION (0X801F0011)

Cause 1: Duplicate filter driver at the same altitude

This error almost always means two filter drivers are trying to register at the same altitude (priority level) on the same volume. Filter drivers sit between apps and the file system — think antivirus, encryption tools, or backup software. When two of them want the same spot, Windows throws this error.

The typical trigger: you install a second antivirus or a new backup tool that uses a filter driver, and it conflicts with an existing driver. I've seen this mostly with people who run both Malwarebytes and Windows Defender, or after installing a third-party encryption tool like BitLocker alternatives.

How to check which drivers are fighting

1. Open Command Prompt as Administrator. Don't skip this — the tool won't work without admin rights. Right-click Start, select Command Prompt (Admin) or Windows PowerShell (Admin).
2. Type fltmc instances and hit Enter. This lists every filter driver instance and its altitude on each volume.
3. Look for two lines that have the same altitude number on the same volume. For example, you might see altitude 370030 on C: twice.
4. Note the instance name and the volume name from those lines.

Fix: remove the duplicate driver

1. In the same admin Command Prompt, type fltmc detach InstanceName Volume. Replace InstanceName with the exact name from the list, and Volume with the volume letter (like C:). Example: fltmc detach MyOldAV C:
2. You should see "Detachment succeeded". If you get an access denied error, the driver is protected — you'll need to uninstall the software that owns it.
3. After detaching, run fltmc instances again. The duplicate entry should be gone. The error should stop appearing.

Cause 2: A leftover driver from uninstalled software

Sometimes you uninstall a program — say an old antivirus — but the filter driver stays behind. The uninstaller missed it. Later, when another driver tries to register at that altitude, you get 0x801F0011.

This happens a lot with Norton, McAfee, or old encryption tools that didn't clean up properly.

Check for orphaned drivers

1. Open Device Manager. Right-click Start, select Device Manager.
2. Click View menu, then Show hidden devices.
3. Expand Non-Plug and Play Drivers.
4. Look for anything with a yellow exclamation or a name you don't recognize — especially things like avc3, mfehidk, or symc8xx (old antivirus remnants).
5. Right-click any suspect driver, choose Properties, then the Driver tab. Check the file version and provider. If it's from software you uninstalled, delete it.
6. To delete: right-click the driver, select Uninstall device, and confirm.

Force removal with fltmc (if Device Manager doesn't work)

1. Run fltmc instances again. Look for instances from software you removed.
2. If you see one, try fltmc detach InstanceName Volume like before.
3. If detach fails, use fltmc unload InstanceName. This removes the instance completely. You'll see "Unload succeeded" if it works.

Cause 3: Third-party filter driver at a reserved altitude

Some filter drivers are supposed to use specific altitude ranges (reserved by Microsoft). If a driver tries to use an altitude that's reserved for something else — like an antivirus trying to use a backup driver's slot — you get the collision error.

I've seen this with older software that wasn't updated for Windows 10 or Windows 11. For example, some encryption tools from 2019 that still use altitudes reserved for newer Windows security features.

Find the altidude range the driver wants

1. Check the software's documentation or support site. Look for the altitude range it uses.
2. Run fltmc instances and compare the altitude numbers to the software's claimed range.
3. If the altitude is outside the range, the driver is likely misconfigured.
4. Contact the software vendor for an updated driver. Or uninstall the old version and install the latest one.

Manual workaround: change the altitude (advanced)

If you can't update the driver, you can manually assign a different altitude. But be careful — this can break the software or cause system instability.

1. Open Registry Editor (regedit).
2. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DriverName\Instances\InstanceName. Replace DriverName and InstanceName with the actual names from fltmc output.
3. Look for a DWORD value called Altitude. Double-click it.
4. Change the number to a different value that's not in use. Check Microsoft's allocated altitudes list first to avoid reserved ranges.
5. Click OK, close regedit, and restart your PC. Run fltmc instances again to verify the collision is gone.

Quick-reference summary table

If none of these work, you might need to run sfc /scannow to check for system file corruption. But honestly, 9 times out of 10 it's just a duplicate filter driver. Start with fltmc instances and you'll find the problem fast.