APC_INDEX_MISMATCH is a Windows Blue Screen of Death (BSOD) stop error associated with an inconsistency in the operating system's Asynchronous Procedure Call (APC) state.

The official bug check value is:

Bug Check: 0x00000001

Stop Code: APC_INDEX_MISMATCH

Microsoft describes Bug Check 0x1 as indicating a mismatch in the asynchronous procedure call state index. The most common underlying condition is a file-system component or driver that performs an unmatched sequence of operations that disable and re-enable APC delivery.

In practical terms, APC_INDEX_MISMATCH is very often a driver or low-level software problem, although system corruption, incompatible hardware, firmware issues, and other kernel-level conflicts should also be investigated.

What Is an APC in Windows?

APC stands for Asynchronous Procedure Call.

Windows uses APCs as part of its thread and kernel architecture. They allow functions to execute asynchronously within the context of a particular thread.

Windows carefully maintains information about whether APC delivery is enabled or disabled.

A kernel-mode driver may temporarily enter a protected or critical region where particular APC operations are disabled. However, the driver must correctly reverse that operation afterward.

For example, Microsoft explains that a call such as:

KeEnterCriticalRegion

must have a corresponding:

KeLeaveCriticalRegion

If a driver enters a critical region but does not correctly leave it—or performs the reverse operation too many times—the APC state can become inconsistent. Windows can detect the mismatch while exiting a system call and deliberately stop the system with APC_INDEX_MISMATCH rather than continue running in an unreliable kernel state.

APC_INDEX_MISMATCH Bug Check Parameters

For advanced troubleshooting, Bug Check 0x1 contains four parameters.

Microsoft defines them broadly as:

Parameter 1: Address of the system function/system call or worker routine.

Parameter 2: Current thread's ApcStateIndex value.

Parameter 3: Current thread's CombinedApcDisable value.

Parameter 4: Type of call:

0 = System call

1 = Worker routine

The CombinedApcDisable information is particularly useful to driver developers and engineers analyzing crash dumps because it incorporates the Special APC and Kernel APC disable state.

For an ordinary Windows user, however, these hexadecimal parameters are usually less useful than determining which driver or kernel component appears in the crash dump.

Common Symptoms of APC_INDEX_MISMATCH

A computer experiencing this problem may show symptoms such as:

• Sudden Blue Screen of Death
• Stop code APC_INDEX_MISMATCH
• Computer restarting unexpectedly
• BSOD when printing
• Crash when connecting a USB device
• BSOD when starting an application
• Crash after installing a Windows update
• Crash after installing or updating a device driver
• BSOD when using audio or video applications
• Crash while using a webcam
• Random Windows freezing followed by restart
• BSOD immediately after signing in
• Repeated crashes during startup
• Crashes when waking the computer from sleep
• BSOD when shutting down or restarting Windows

The exact activity occurring immediately before the BSOD can provide an important clue.

For example, if the computer crashes every time you print, investigating the printer driver should receive much higher priority than replacing RAM.

What Causes APC_INDEX_MISMATCH?

1. Faulty or Incompatible Device Drivers

Drivers are among the most important suspects.

Microsoft states that the most common cause of this bug check involves a file system or driver having mismatched calls associated with disabling and re-enabling APCs.

Potentially involved drivers include:

• Printer drivers
• Audio drivers
• Graphics drivers
• Chipset drivers
• USB drivers
• Bluetooth drivers
• Wi-Fi drivers
• Ethernet drivers
• Webcam drivers
• Storage controller drivers
• Scanner drivers
• VPN network drivers
• Security software drivers

A driver does not necessarily have to be completely broken. It may simply be incompatible with a newer Windows build.

2. Recently Updated Drivers

If APC_INDEX_MISMATCH started immediately after updating a driver, the newer driver should be treated as a prime suspect.

A driver can install successfully but still contain a bug that appears only under particular workloads.

Examples include crashes occurring only while:

• Printing
• Playing audio
• Connecting Bluetooth devices
• Using a webcam
• Gaming
• Entering sleep mode
• Resuming from sleep

In such situations, rolling back the recently updated driver can be more effective than repeatedly reinstalling the same version.

3. Windows Update and Driver Compatibility

Sometimes APC_INDEX_MISMATCH appears after a Windows cumulative, feature, or driver update.

This does not automatically mean Windows itself is defective.

An operating-system update can expose an incompatibility in an existing third-party driver.

Therefore, investigate both:

Windows update history

and

recent driver changes

before removing updates.

4. Printer Driver Problems

Printing deserves special attention when diagnosing this stop code.

Historical Windows incidents have shown APC_INDEX_MISMATCH occurring during printing, including cases involving win32kfull.sys.

If the BSOD occurs specifically when printing, investigate:

• Printer driver
• Print server driver
• Universal printer driver
• Old Type 3 printer drivers
• Recently installed printer software
• Windows printing updates
• Print spooler-related software

Try removing the printer and installing the manufacturer's current supported driver.

5. Audio Drivers

Audio drivers and related utilities operate closely with Windows hardware and can introduce kernel-level conflicts.

If crashes occur during:

• Video conferencing
• Music playback
• Gaming
• Connecting headphones
• Changing audio devices
• Using USB audio devices

check the audio driver.

Instead of relying only on Device Manager's automatic search, obtain the correct driver from the PC or motherboard manufacturer whenever possible.

6. Graphics Drivers

Display drivers operate in kernel mode and can cause severe crashes when corrupted or incompatible.

Check graphics drivers from:

• NVIDIA
• AMD
• Intel
• Laptop manufacturer
• PC manufacturer

If the BSOD began after a GPU driver update, consider rolling back to the previously stable release.

7. Third-Party Security Software

Antivirus, endpoint security, firewall, encryption, anti-ransomware and monitoring applications can install kernel-mode components.

A buggy or incompatible security driver can therefore contribute to BSOD problems.

If APC_INDEX_MISMATCH started immediately after installing or upgrading security software, investigate that product carefully.

Do not leave a production computer permanently unprotected merely as a troubleshooting method. Use temporary isolation only when appropriate and restore suitable protection afterward.

8. VPN and Network Filter Drivers

VPN applications commonly install virtual network adapters and filtering drivers.

Potential suspects include:

• VPN clients
• Network monitoring tools
• Packet filtering software
• Endpoint security products
• Traffic-management utilities
• Virtual network adapters

Update or temporarily uninstall recently added network software if the crashes correlate with network activity.

9. Corrupted Windows System Files

Although drivers are a primary suspect, damaged Windows files can complicate the situation.

System corruption can result from:

• Interrupted updates
• Improper shutdown
• Storage problems
• Malware
• Failed software installation
• Disk errors

Windows includes SFC and DISM tools that can help verify and repair system components.

How to Fix APC_INDEX_MISMATCH

Step 1: Identify What Changed Recently

Before making major changes, ask:

When did the BSOD start?

Then check whether you recently:

• Installed Windows updates
• Updated a driver
• Installed a printer
• Installed a new GPU
• Added RAM
• Installed antivirus software
• Installed VPN software
• Connected a new USB device
• Updated BIOS
• Installed motherboard utilities

If the problem began immediately after one of these events, start troubleshooting there.

Step 2: Update Windows

Open:

Settings → Windows Update

Select:

Check for updates

Install applicable Windows quality and security updates.

Restart the computer afterward.

This is particularly important if the problem is caused by a known Windows compatibility issue that has subsequently been corrected.

Step 3: Update Device Drivers

Prioritize drivers for:

1. Chipset
2. Storage controller
3. Graphics
4. Audio
5. Network
6. Wi-Fi
7. Bluetooth
8. Printer
9. Webcam
10. USB-related hardware

Whenever possible, obtain drivers from the PC, motherboard, or device manufacturer's official support channel.

Avoid random third-party "driver updater" websites.

Step 4: Roll Back a Recently Updated Driver

If the BSOD started after a driver update:

Press:

Windows + X

Select:

Device Manager

Locate the relevant device.

Right-click it and select:

Properties → Driver → Roll Back Driver

Restart Windows and test again.

If Roll Back Driver is unavailable, manually install an earlier known-good driver obtained from the manufacturer.

Step 5: Run System File Checker

Open Command Prompt or Windows Terminal as Administrator.

Run:

DISM /Online /Cleanup-Image /RestoreHealth

Allow the operation to complete.

Then run:

sfc /scannow

Restart Windows afterward.

SFC verifies protected Windows system files and attempts to replace corrupted versions, while DISM can repair the Windows component store used for servicing.

Step 6: Check the File System

Open Command Prompt as Administrator and run:

chkdsk C: /scan

For problems requiring an offline repair, Windows may require:

chkdsk C: /f

If the volume is currently in use, Windows may offer to schedule the repair for the next restart.

Back up important information before performing extensive disk repair on a drive suspected of physically failing.

Step 7: Check Event Viewer

Microsoft specifically recommends checking the System log in Event Viewer for additional errors that may identify the device or driver involved in Bug Check 0x1.

Press:

Windows + R

Enter:

eventvwr.msc

Navigate to:

Windows Logs → System

Look for critical and error events immediately before the BSOD.

Pay particular attention to:

• Driver errors
• Disk errors
• Device failures
• Service crashes
• WHEA hardware errors
• Display driver problems

Do not assume every error shown around the same time caused the BSOD. Event Viewer provides clues that need to be correlated with the crash.

Step 8: Check Reliability Monitor

Windows Reliability Monitor provides an easier timeline for many users.

Press:

Windows + R

Enter:

perfmon /rel

Look for red failure icons around the date and time of the crash.

Review:

• Windows failures
• Application failures
• Hardware errors
• Driver installation events
• Windows updates

This can help establish whether crashes started immediately after a particular software or driver change.

Step 9: Boot Windows in Safe Mode

Safe Mode loads Windows with a reduced collection of drivers and services.

If APC_INDEX_MISMATCH disappears completely in Safe Mode, a third-party driver, application or service becomes a stronger suspect.

You can access Windows Recovery and navigate through:

Troubleshoot → Advanced options → Startup Settings → Restart

Then choose the appropriate Safe Mode option.

Step 10: Perform a Clean Boot

A clean boot is useful when Windows starts normally but third-party software may be responsible.

Run:

msconfig

Open the Services tab.

Select:

Hide all Microsoft services

Then disable appropriate third-party services for diagnostic testing.

Also disable unnecessary startup applications using Task Manager.

Restart and test the system.

If the problem disappears, re-enable services gradually until the conflicting software is identified.

Step 11: Test RAM

Memory problems can cause many types of seemingly unrelated BSODs.

Press:

Windows + R

Enter:

mdsched.exe

Select:

Restart now and check for problems

Windows Memory Diagnostic will test memory during reboot.

For persistent or suspicious hardware-related crashes, a longer dedicated memory test may be appropriate.

Step 12: Disconnect Recently Added Hardware

Temporarily disconnect nonessential external devices such as:

• USB hubs
• External drives
• Printers
• Scanners
• Webcams
• USB sound devices
• Bluetooth adapters
• Docking stations

Restart Windows.

Reconnect devices individually.

If the BSOD returns after connecting a particular device, investigate that device and its driver.

Step 13: Investigate win32kfull.sys Carefully

Some APC_INDEX_MISMATCH crashes may mention:

win32kfull.sys

Do not immediately download a replacement win32kfull.sys file from the Internet.

The Windows module shown on a blue screen or in a dump is not necessarily the component that originally caused the corruption.

For example, a third-party printer driver can interact with Windows graphics/printing components and cause the eventual crash to surface in a Microsoft system module.

If win32kfull.sys appears while printing, focus strongly on the printer driver and recent Windows/printing changes.

Step 14: Investigate ntoskrnl.exe Correctly

Crash-analysis utilities sometimes report:

ntoskrnl.exe

This is the Windows NT kernel.

Seeing ntoskrnl.exe does not automatically mean that Windows itself is defective.

A third-party driver can corrupt kernel state and the Windows kernel later detects the inconsistency.

Therefore, the stack trace and complete crash dump are much more useful than simply searching for the last .sys filename displayed.

Step 15: Analyze Minidump Files

Windows commonly stores small crash dumps in:

C:\Windows\Minidump

Typical files look like:

082926-12345-01.dmp

Crash dumps can provide valuable information about:

• Bug check code
• Crash parameters
• Active process
• Call stack
• Loaded modules
• Suspected driver
• Failure bucket

Microsoft notes that bug-check dump files can contain additional memory-state information useful for determining why the operating system stopped.

Advanced Diagnosis with WinDbg

IT professionals and advanced users can use Microsoft's WinDbg debugger.

After loading the dump, a common first command is:

!analyze -v

Microsoft specifically identifies the !analyze debugger extension as useful for determining the root cause of APC_INDEX_MISMATCH.

The APC debugger extension can also be used when deeper analysis of APC state is required.

When analyzing the output, examine information such as:

• BUGCHECK_CODE
• MODULE_NAME
• IMAGE_NAME
• PROCESS_NAME
• FAILURE_BUCKET_ID
• STACK_TEXT
• SYMBOL_NAME

Do not rely solely on IMAGE_NAME.

Look at the complete call stack and surrounding circumstances.

Driver Verifier: Use with Caution

Windows Driver Verifier can stress kernel-mode drivers to expose faulty behavior.

Run:

verifier

However, Driver Verifier should not be the first troubleshooting step on an important production computer.

It can deliberately trigger additional crashes when it detects incorrect driver behavior.

Before using Driver Verifier:

• Back up important data.
• Create a restore point if practical.
• Know how to enter Safe Mode or Windows Recovery.
• Know how to disable Driver Verifier.

To turn Driver Verifier off from an elevated command prompt:

verifier /reset

Restart Windows afterward.

What If Windows Cannot Boot?

If APC_INDEX_MISMATCH creates a boot loop, enter the Windows Recovery Environment.

Navigate to:

Troubleshoot → Advanced options

Depending on the circumstances, useful options may include:

• Startup Settings
• System Restore
• Uninstall Updates
• Startup Repair
• Command Prompt

If the crash began immediately after installing a driver, Safe Mode may allow you to remove or roll back that driver.

Should You Uninstall the Latest Windows Update?

Not automatically.

First determine whether the crash started immediately after the update and whether a specific driver is actually responsible.

Consider uninstalling an update when:

• The BSOD began directly after that update.
• Multiple machines show the same behavior.
• The affected driver is already current.
• A known compatibility problem exists.
• Normal driver troubleshooting has failed.

Afterward, continue checking for a corrected Windows update or updated device driver.

Should You Update the BIOS?

BIOS/UEFI updates can improve:

• Hardware compatibility
• CPU microcode
• Memory compatibility
• Power management
• Device initialization
• Platform stability

But BIOS updating is not a universal solution for APC_INDEX_MISMATCH.

Update firmware only when appropriate for the machine and follow the manufacturer's instructions carefully.

An interrupted or incorrect firmware update can make a computer unbootable.

Is APC_INDEX_MISMATCH a Hardware Failure?

Not necessarily.

Because Bug Check 0x1 is strongly associated with incorrect driver/APC state management, driver troubleshooting should normally receive high priority.

Hardware testing becomes more important when:

• Multiple unrelated stop codes occur.
• Windows crashes during installation.
• Memory diagnostics report errors.
• WHEA hardware errors appear.
• Storage SMART diagnostics show problems.
• The computer freezes even outside Windows.
• Crashes continue after a clean Windows installation.

Recommended Troubleshooting Order

For most systems, use this sequence:

1. Note exactly when the BSOD occurs.

2. Check recently installed Windows updates, software and hardware.

3. Update or roll back the suspected device driver.

4. Install applicable Windows updates.

5. Run DISM and SFC.

6. Check Event Viewer and Reliability Monitor.

7. Test in Safe Mode or perform a clean boot.

8. Test RAM and storage if necessary.

9. Analyze C:\Windows\Minidump files.

10. Use WinDbg for deeper diagnosis.

11. Consider Driver Verifier only when normal diagnosis cannot identify the faulty driver.

This approach is generally preferable to randomly reinstalling drivers or replacing hardware.

How to Prevent APC_INDEX_MISMATCH in the Future

Keep Windows updated, but also maintain current manufacturer-supported drivers.

Avoid installing unnecessary:

• Driver updater programs
• Registry cleaners
• Kernel-level optimization tools
• Unknown antivirus applications
• Unsupported hardware utilities

Before major driver or firmware changes:

• Back up important data.
• Create a restore point where appropriate.
• Record the existing driver version.
• Download the known-good driver before replacing it.

For business-critical systems, test major driver and Windows updates on a limited number of machines before broad deployment whenever possible.

Frequently Asked Questions (FAQ)

1. What does APC_INDEX_MISMATCH mean?

APC_INDEX_MISMATCH means Windows detected an inconsistency in the asynchronous procedure call state of a thread. The official bug check code is 0x00000001.

2. Is APC_INDEX_MISMATCH a serious error?

Yes. It is a kernel-level stop condition that forces Windows to stop to prevent the system from continuing in an inconsistent state. An isolated crash may be temporary, but repeated occurrences should be investigated.

3. What is the stop code for APC_INDEX_MISMATCH?

The bug check value is:

0x00000001

It is also known as Bug Check 0x1.

4. What is the most common cause?

According to Microsoft, the most common cause is a file system or driver having mismatched calls related to disabling and re-enabling APCs.

5. Can a printer driver cause APC_INDEX_MISMATCH?

Yes. Printer-related driver problems have historically been associated with APC_INDEX_MISMATCH crashes, including crashes involving win32kfull.sys.

6. Why does APC_INDEX_MISMATCH happen only when I print?

This strongly suggests investigating the printer driver, print-related software, and recent Windows or printer-driver changes.

7. Can an audio driver cause this BSOD?

Yes. Kernel-level audio drivers can potentially cause stop errors if they are faulty or incompatible.

8. Can a graphics driver cause APC_INDEX_MISMATCH?

Potentially. If crashes began after a GPU driver update or happen during graphics-intensive operations, update or roll back the graphics driver as part of troubleshooting.

9. What does win32kfull.sys mean in an APC_INDEX_MISMATCH crash?

win32kfull.sys is a Windows system component. Its presence in a crash does not automatically prove that the file itself is defective. A third-party driver interacting with Windows can be the underlying cause.

10. Should I download win32kfull.sys from the Internet?

No. Do not replace Windows system files with copies downloaded from random DLL/SYS websites. Use Windows servicing tools such as DISM and SFC instead.

11. Can Windows Update cause APC_INDEX_MISMATCH?

A problem can appear after an update, either because of the update itself or because the update exposes an incompatibility with an existing driver.

12. Can RAM cause this error?

Memory problems can contribute to BSODs, although APC_INDEX_MISMATCH should first prompt careful investigation of drivers and kernel-level software.

13. Where are Windows BSOD minidumps stored?

They are commonly stored in:

C:\Windows\Minidump

provided Windows is configured to create small memory dumps.

14. How do I analyze APC_INDEX_MISMATCH?

Use Microsoft WinDbg and begin with:

!analyze -v

Then examine the stack, modules, processes and failure information. Microsoft specifically recommends WinDbg as an advanced diagnostic approach for Bug Check 0x1.

15. Can antivirus software cause this error?

Potentially. Some security products install kernel-mode drivers, so an incompatible or defective security driver can contribute to BSODs.

16. Should I reinstall Windows immediately?

Usually not. First investigate drivers, recent changes, system-file integrity, Event Viewer and crash dumps. Reinstallation should generally be considered only after reasonable diagnosis and repair options have failed.

17. Can Safe Mode help diagnose the problem?

Yes. If the computer works reliably in Safe Mode, a third-party driver, service or application becomes a stronger suspect.

18. Does ntoskrnl.exe mean my Windows kernel is corrupted?

Not necessarily. ntoskrnl.exe is central to Windows kernel operation and frequently appears in crash analysis. Another driver may have created the condition that the kernel ultimately detected.

19. Should I use Driver Verifier?

Driver Verifier can help identify defective drivers, but it is an advanced diagnostic tool and may intentionally cause additional BSODs when violations are detected. Use it carefully and know how to run verifier /reset if recovery is required.

20. Can APC_INDEX_MISMATCH be permanently fixed?

Yes, in many cases. Once the defective or incompatible driver, software component, system corruption, or hardware problem is correctly identified and corrected, the crashes should stop.

Conclusion

APC_INDEX_MISMATCH (Bug Check 0x00000001) is a Windows kernel-level BSOD indicating that Windows detected an inconsistent APC state. Microsoft identifies mismatched APC-disable/enable handling by a driver or file-system component as the most common cause.

For most users and administrators, the best strategy is therefore not to immediately reinstall Windows or replace hardware. Start by identifying what changed immediately before the crashes began, especially device drivers, printers, Windows updates, security products and newly installed hardware.

Update or roll back suspicious drivers, repair Windows with DISM and SFC, review Event Viewer and Reliability Monitor, test Safe Mode, and examine the crash dumps in C:\Windows\Minidump.

For persistent cases, WinDbg analysis can provide substantially better evidence about the offending driver or kernel component.

Official technical reference: Microsoft Learn — Bug Check 0x1: APC_INDEX_MISMATCH

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