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Windows Stop Code 0x000000A4: CNSS_FILE_SYSTEM_FILTER

Quick Answer CNSS_FILE_SYSTEM_FILTER (Stop Code 0x000000A4) is a Windows Blue Screen of Death (BSOD) error associated with a problem in the CNSS file system ...

Bison Technical Team Enterprise IT specialists

Updated 03 Nov 2026 10 min read 0 total views

Structured technical guidanceSafety notes included where requiredSources listed below

Quick Answer

CNSS_FILE_SYSTEM_FILTER (Stop Code 0x000000A4) is a Windows Blue Screen of Death (BSOD) error associated with a problem in the CNSS file system filter. Microsoft identifies low or exhausted nonpaged pool memory as a possible cause. This can occur when file indexing or another kernel-mode operation requires memory that is no longer sufficiently available.

The error does not automatically indicate defective RAM or a particular faulty driver. Start by checking recent driver and software changes, monitoring nonpaged pool memory, and examining Windows crash dump files. If the problem continues, use diagnostic tools such as PoolMon and WinDbg to investigate the underlying cause.

What Is CNSS_FILE_SYSTEM_FILTER?

CNSS_FILE_SYSTEM_FILTER is the name associated with Windows bug check 0xA4. A bug check is a critical operating-system error that forces Windows to stop when it encounters a condition that prevents safe continued operation.

A file system filter is a kernel-mode component that can monitor or participate in file-system operations. These components are used by Windows features and third-party applications that interact with files, storage, security, backup and indexing.

Microsoft documents nonpaged pool memory depletion as a possible reason for this particular bug check.

Understanding Nonpaged Pool Memory

Nonpaged pool is a section of kernel-mode memory that must remain resident in physical RAM. Windows drivers and kernel components use it to store information that must remain immediately accessible and cannot be moved to the paging file.

When kernel-mode components allocate memory without releasing it properly, available nonpaged pool memory can decrease. If another component requires memory but sufficient resources are unavailable, Windows may encounter a critical failure.

Common Causes of CNSS_FILE_SYSTEM_FILTER (0xA4)

1. Nonpaged Pool Memory Exhaustion

Excessive kernel memory allocation can consume available nonpaged pool resources. This can happen when a driver or kernel component allocates memory repeatedly without releasing previously allocated memory.

2. File Indexing Activity

Microsoft specifically identifies file indexing as a situation in which nonpaged pool memory may become very low. Another kernel-mode driver requesting memory during this activity may contribute to the bug check.

3. Driver Memory Leaks

A driver memory leak occurs when a driver allocates memory but fails to release it correctly. Over time, these allocations may accumulate and consume available kernel memory.

Storage, security, backup and file-system filter drivers may be relevant investigation targets, but their presence does not prove that they caused the crash.

4. Insufficient Available Physical Memory

Physical RAM limitations can contribute to memory pressure. Microsoft identifies adding physical memory as one possible resolution when nonpaged pool memory depletion is involved.

However, additional RAM will not necessarily solve a driver memory leak or another underlying software problem.

Symptoms of the 0x000000A4 Blue Screen

Windows displays a blue screen with the stop code 0x000000A4.
The computer automatically restarts after a critical system error.
Crashes occur during file indexing or other file-system operations.
System instability develops during heavy storage or application activity.
Windows generates crash dump files after a system failure, if dump collection is configured.

These symptoms are useful diagnostic clues but do not independently establish the root cause.

Understanding Bug Check Parameters

Microsoft documents four parameters for bug check 0xA4.

Parameter	Meaning
Parameter 1	Contains source file identifier and source line information.
Parameter 2	Reserved.
Parameter 3	Reserved.
Parameter 4	Reserved.

The first parameter contains two 16-bit values:

High 16 bits: Source file identifier.
Low 16 bits: Source line number.

These values are primarily useful in an appropriate debugging and source-code context. They should not be interpreted as direct identification of a faulty hardware component or third-party driver.

How to Fix CNSS_FILE_SYSTEM_FILTER (0x000000A4)

Follow these troubleshooting procedures in order. Start with simple checks before attempting advanced driver or memory diagnostics.

Step 1: Restart Windows and Record the Error

If the error occurred only once and Windows has restarted successfully, monitor the system before making major changes.

If it occurs again, record:

The complete stop code and error message.
All four bug-check parameters, if available.
Windows edition, version and build number.
Recently installed software and drivers.
Whether the crash occurs during startup, file indexing, backup or another identifiable activity.

Step 2: Disconnect Recently Added Hardware

If the problem started after connecting new hardware, shut down the computer and disconnect the recently added peripheral if it can be safely removed.

Examples include external storage devices, USB peripherals and recently installed expansion cards.

Restart Windows and check whether the error continues. This is a diagnostic test rather than a confirmed fix for 0xA4.

Step 3: Start Windows in Safe Mode

Safe Mode starts Windows with a limited set of essential drivers and services. It can help determine whether a recently installed application or driver is involved.

Open Settings.
Navigate to System > Recovery.
Under Advanced startup, select Restart now.
Select Troubleshoot in Windows Recovery Environment.
Open Advanced options > Startup Settings.
Select Restart.
Press F4 for Safe Mode or F5 for Safe Mode with Networking.

Check whether the blue screen occurs in Safe Mode. If it does not, a driver, service or application that is not loaded in Safe Mode becomes a possible investigation target.

For additional information, see Microsoft Windows Startup Settings.

Step 4: Check Recently Updated Drivers

Kernel-mode drivers can contribute to memory-pool problems when they allocate or release memory incorrectly.

Right-click Start and open Device Manager.
Expand the relevant hardware category.
Right-click the suspected device and select Properties.
Open the Driver tab.
Review the driver provider, version and installation date.
If the problem began after a driver update, consider using Roll Back Driver when available.

Download driver updates only from Windows Update or the relevant hardware manufacturer's official website.

Do not remove unrelated drivers simply because they appear in Device Manager.

Step 5: Install Available Windows Updates

Open Settings.
Select Windows Update.
Select Check for updates.
Install applicable updates.
Restart the computer if requested.

If the problem began immediately after a particular update, record its KB number and installation date. Consider a supported rollback only when the timing and diagnostic evidence justify it.

Step 6: Monitor Nonpaged Pool Memory

Monitoring nonpaged pool usage is important because memory depletion is a documented possible cause of this bug check.

Using Task Manager

Press Ctrl + Shift + Esc.
Open the Performance tab.
Select Memory.
Locate the Paged pool and Non-paged pool values.
Monitor the values during normal system operation.

Observe whether nonpaged pool usage continually increases. A high value alone does not establish a memory leak.

Using Performance Monitor

Press Win + R.
Type perfmon and press Enter.
Open Performance Monitor.
Add the Memory > Pool Nonpaged Bytes counter.
Record memory usage over time.

Compare the measurements with the activities being performed on the computer. A steadily increasing value may indicate a memory leak and requires further investigation.

Step 7: Use PoolMon to Investigate Memory Usage

PoolMon is Microsoft's Memory Pool Monitor utility, included with the Windows Driver Kit (WDK). It helps IT professionals investigate kernel memory allocations grouped by pool tag.

To use PoolMon:

Obtain PoolMon through the official Windows Driver Kit.
Open an elevated Command Prompt.
Start PoolMon.
Press P to select nonpaged pool information.
Press B to sort entries by byte usage.
Monitor allocation behaviour over time.

A pool tag with continually increasing memory usage may provide a clue about a possible memory leak.

To display mapped driver information, Microsoft's documentation describes using a pool-tag mapping file with the following command:

poolmon /g "C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\triage\pooltag.txt"

The path is an example and must be adjusted to the actual installation. The command requires PoolMon and the corresponding mapping file.

PoolMon results require technical interpretation. A large allocation does not automatically mean that a driver is defective.

Official documentation:

Step 8: Analyse Windows Crash Dump Files

If the blue screen continues, crash dump analysis can help identify the circumstances surrounding the failure.

Common Windows dump locations include:

C:\Windows\Minidump - Small memory dump files.
C:\Windows\MEMORY.DMP - Default location for a kernel or complete memory dump, depending on configuration.

Using WinDbg

WinDbg is Microsoft's debugging tool for analysing Windows crash dumps.

Install WinDbg using Microsoft's official distribution.
Open WinDbg.
Select File > Open dump file.
Select the relevant .dmp file.
Wait for the debugger to load the dump and required symbols.
Run the following command:

!analyze -v

This command requests detailed automatic crash analysis.

To display the bug-check code and parameters, use:

.bugcheck

Review the bug-check information, stack trace and loaded modules together. A driver listed in a crash report is not automatically the root cause.

Crash dump files may contain sensitive information from system memory. Review them before sharing them with anyone.

See Microsoft's WinDbg Kernel-Mode Dump Analysis Guide.

Step 9: Check Whether Additional RAM Is Needed

If diagnostic evidence indicates that available memory resources are insufficient, additional physical RAM may help.

Before purchasing memory, check:

Motherboard or laptop memory capacity.
Supported memory generation and speed.
Available memory slots.
Module compatibility.
Whether the memory is replaceable or soldered.

Additional RAM does not necessarily correct a defective driver or kernel memory leak.

Advanced Troubleshooting for IT Administrators

Monitor Kernel Memory Counters

Performance Monitor can help administrators investigate memory usage over time.

Useful counters include:

Memory > Pool Nonpaged Bytes.
Memory > Pool Paged Bytes.
Paging File > % Usage.

Correlate memory changes with indexing, backup, security scanning and other system activities.

See Microsoft's guidance on determining whether a memory leak exists.

Identify Installed File-System Filter Drivers

Third-party security, backup, encryption and storage-management products may install file-system filter drivers.

To inspect registered file-system filters, open an elevated Command Prompt or PowerShell window and run:

fltmc filters

This command displays registered file-system filter drivers and associated information.

Review the output alongside crash dump evidence and installed software. The presence of a particular filter does not establish that it caused the crash.

Do not manually unload or remove unfamiliar filters from a production computer. Use the relevant vendor's supported repair or uninstall procedure when a component has been identified as a likely contributor.

Use Driver Verifier Carefully

Driver Verifier is an advanced Windows diagnostic utility that monitors selected drivers for certain illegal operations and errors.

It can deliberately cause additional blue screens when it detects driver violations. Do not enable it indiscriminately on an unstable computer.

Preserve existing crash dumps and collect memory-pool evidence first. Use Driver Verifier only when a qualified technician has identified a specific driver to investigate and has prepared a recovery procedure.

Recovery If Windows Cannot Start

If the 0xA4 error prevents Windows from starting normally, use Windows Recovery Environment.

Allow Windows to enter Automatic Repair, or start the computer using supported Windows recovery media.
Select Troubleshoot.
Open Advanced options.
Try Startup Settings and Safe Mode.
If the problem began after a recent update, consider the supported update-uninstall option.
Use System Restore if a suitable restore point exists and other options are insufficient.

Protect important files before attempting recovery operations whenever possible. If BitLocker is enabled, make sure the recovery key is available.

For additional information, see Windows Recovery Options.

Frequently Asked Questions

What does CNSS_FILE_SYSTEM_FILTER 0x000000A4 mean?

It is a Windows bug check indicating a problem in the CNSS file system filter. Microsoft identifies nonpaged pool memory depletion as a possible cause.

Is this error caused by faulty RAM?

Not necessarily. Memory-pool exhaustion can occur because of kernel-mode driver behaviour or insufficient available memory. The stop code alone does not establish physical RAM failure.

Can outdated drivers cause this blue screen?

A kernel-mode driver that improperly allocates or releases memory may contribute to memory-pool depletion. However, an outdated driver is not a confirmed cause of every 0xA4 crash.

Can file indexing trigger this error?

Microsoft specifically mentions file indexing as a situation in which available nonpaged pool memory may become very low. Another kernel-mode driver requesting memory at that time may trigger the bug check.

Can adding more RAM fix the problem?

Additional physical memory may help when available memory is insufficient. However, it will not necessarily resolve a kernel-mode driver memory leak.

How can I identify the problematic driver?

Use WinDbg to analyse crash dumps and PoolMon to investigate nonpaged pool allocation patterns. These tools provide diagnostic evidence but may require further investigation to establish the actual cause.

Should I reinstall Windows to fix this error?

Reinstalling Windows should not be the first troubleshooting step. Investigate driver changes, memory-pool usage and crash dump evidence before considering an operating-system reinstallation.

Sources

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