CACHE_INITIALIZATION_FAILED (0x00000066) – Windows Blue Screen Error

Quick Answer

CACHE_INITIALIZATION_FAILED (Bug Check 0x00000066) is a rare Windows Blue Screen of Death (BSOD) error that indicates a failure during cache initialization in the Windows startup process. Microsoft identifies this stop code as an initialization failure that occurs very infrequently.

The exact underlying cause cannot be determined from the stop code alone. If Windows displays this error, investigate recent hardware or software changes, review crash dump information, and use Windows Recovery Environment (WinRE) if the computer cannot start normally. Avoid assuming that the physical CPU cache or a particular storage device is defective without diagnostic evidence.

What Does CACHE_INITIALIZATION_FAILED Mean?

Windows performs several initialization operations when starting. These operations prepare the kernel and its supporting components so that the operating system can function correctly.

The bug check 0x00000066 identifies a failure associated with cache initialization. It is an uncommon stop code, and Microsoft's public documentation does not provide a detailed list of causes or a universal repair procedure for it.

The word cache in this stop-code name should not automatically be interpreted as a problem with the processor's L1, L2, or L3 cache. The stop code alone does not identify which component or internal operation failed.

Symptoms

A computer affected by this error may exhibit one or more of the following symptoms:

• A blue screen displaying CACHE_INITIALIZATION_FAILED.
• The stop code 0x00000066 appearing during Windows startup.
• Windows restarting automatically after displaying the error.
• Repeated startup failures or an inability to reach the sign-in screen.
• A crash following a recent driver, software, hardware, or firmware change.
• A crash dump being generated when Windows successfully writes diagnostic information.

These are possible symptoms of a startup failure, not proof of a particular root cause.

Possible Causes

Microsoft describes this bug check as very infrequent but does not publish a definitive cause list for it. The following are diagnostic areas worth investigating rather than confirmed causes of every 0x00000066 crash.

• Windows system corruption: Damaged or inconsistent operating-system components may interfere with startup.
• Driver or software changes: A recently installed or updated low-level driver or system utility may be relevant if the crash began immediately afterward.
• Hardware instability: Unstable memory, motherboard, processor, or other hardware may contribute to system initialization failures.
• Firmware or configuration changes: Recent BIOS/UEFI updates, firmware changes, or altered hardware settings may be relevant.
• Storage problems: Disk errors or an inaccessible system volume may complicate startup and should be checked when other symptoms point to storage.

Before You Begin

• Record the complete stop code and any additional error information displayed on the blue screen.
• Note whether the problem started after a Windows update, driver installation, hardware replacement, or firmware change.
• Disconnect recently connected nonessential USB devices and peripherals before another startup attempt.
• If Windows still starts occasionally, back up important personal files before performing recovery operations.
• Keep your BitLocker recovery key available if device encryption is enabled. Do not share the recovery key with anyone.

How to Troubleshoot CACHE_INITIALIZATION_FAILED (0x00000066)

1. Disconnect Nonessential External Devices

Remove external storage devices, USB hubs, printers, docking stations, and other nonessential peripherals. Leave only the keyboard, mouse, display, and hardware required to start the computer.

1. Shut down the computer.
2. Disconnect nonessential external devices.
3. Turn on the computer and check whether Windows starts.
4. If Windows starts successfully, reconnect devices individually to identify whether a particular device or its driver is associated with the problem.

This test helps isolate external-device or driver-related problems. It does not establish that a peripheral caused the original bug check.

2. Start Windows in Safe Mode

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

1. Enter Windows Recovery Environment. If Windows fails to start repeatedly, Windows may automatically display recovery options. Otherwise, use Windows installation or recovery media.
2. Select Troubleshoot.
3. Select Advanced options.
4. Select Startup Settings.
5. Select Restart.
6. After the restart, press 4 or F4 for Safe Mode. Select 5 or F5 if Safe Mode with Networking is required.

If Safe Mode works, investigate recently installed drivers, system utilities, and software. Remove or roll back only changes that are relevant to when the crashes began.

If Windows cannot reach Safe Mode, continue with the recovery options below.

3. Run Startup Repair

Startup Repair is a built-in Windows Recovery Environment tool designed to diagnose and repair certain problems that prevent Windows from starting.

1. Start the computer in Windows Recovery Environment.
2. Select Troubleshoot.
3. Open Advanced options.
4. Select Startup Repair.
5. Choose the Windows installation if prompted.
6. Allow the diagnostic process to finish and follow the displayed instructions.

Restart the computer and check whether Windows starts. Startup Repair does not fix every type of kernel initialization failure, so further investigation may be necessary.

4. Check and Repair Windows System Files

If Windows starts normally or in Safe Mode, use System File Checker (SFC) to check protected Windows system files.

Open Command Prompt or Windows Terminal as an administrator and execute:

sfc /scannow

Wait for the scan to complete. The result indicates whether Windows found integrity violations and whether it could repair them.

If SFC reports that some files could not be repaired, use the Deployment Image Servicing and Management (DISM) tool to check and repair the Windows component store. Open an elevated Command Prompt and run:

DISM /Online /Cleanup-Image /RestoreHealth

After DISM finishes, run SFC again:

sfc /scannow

Restart Windows and check whether the error returns.

Important: The commands above use /Online and are intended for a running Windows installation. They are not directly interchangeable with commands used to service an offline Windows installation from recovery media.

5. Check the System Drive

If there are additional symptoms such as slow disk access, storage-related warnings, file corruption, or unusual drive behaviour, check the Windows system drive.

Open Command Prompt as an administrator and run:

chkdsk C: /scan

This performs an online scan of the C: volume for file-system problems. If Windows is installed on another drive letter, replace C: with the correct volume letter.

Review the scan results before deciding whether an offline repair is required. Do not use repair or formatting options without first considering the risk to important data.

6. Review Recent Driver and Windows Updates

If the error began after a particular update or driver installation, investigate that change before attempting broader system modifications.

1. Open Device Manager if Windows can start.
2. Locate the device whose driver was recently changed.
3. Open its Properties and select the Driver tab.
4. Use Roll Back Driver if the option is available and the problem started after that driver update.
5. Alternatively, install a compatible driver obtained from the computer or hardware manufacturer's official website.

For a recent Windows update, use the Windows Recovery Environment option under Troubleshoot > Advanced options to access Uninstall Updates, if available. Choose the relevant update type and follow the instructions.

Do not remove unrelated drivers or security updates without a specific diagnostic reason.

7. Check Hardware Stability

If the crash continues after software and recovery checks, investigate hardware stability.

• Run the computer manufacturer's built-in hardware diagnostics, if available.
• Test system memory using Windows Memory Diagnostic or an appropriate manufacturer-provided diagnostic tool.
• Check for overheating, unexpected shutdowns, and hardware warning indicators.
• Review recent RAM, motherboard, processor, and storage changes.
• If the problem began after changing memory profiles, overclocking, or firmware settings, investigate those changes using the manufacturer's documented recovery procedure.

Hardware inspection and component replacement should be performed by a qualified technician when the required knowledge or equipment is unavailable.

Advanced Troubleshooting for IT Professionals

Analyze the Windows Crash Dump

If Windows generated a crash dump, examine it to identify the failure context. Dump files may be located in the following standard locations:

• C:\Windows\Minidump – Small memory dump files, when configured and successfully generated.
• C:\Windows\MEMORY.DMP – A kernel or complete memory dump, depending on the configured dump type.

Use WinDbg from Microsoft to open the dump file and inspect the bug check information. A useful initial debugger command is:

!analyze -v

Review the bug check code, parameters, stack information, loaded modules, and any additional diagnostic information. The dump may not contain enough information to establish the root cause, particularly for an uncommon early-startup failure.

Do not identify a driver as defective solely because its name appears in the loaded-module list. Correlate the dump with the crash timeline and other diagnostic evidence.

Inspect Bug Check Parameters

The Microsoft documentation identifies 0x00000066 as CACHE_INITIALIZATION_FAILED but does not publish a detailed parameter interpretation for this stop code on its dedicated documentation page. If parameters are displayed in a dump or diagnostic report, preserve their exact values for further debugging rather than assigning unsupported meanings to them.

What If Windows Cannot Start?

If normal startup and Safe Mode are unavailable, use Windows Recovery Environment in the following order:

1. Try Startup Repair.
2. Use System Restore if a suitable restore point exists.
3. Consider uninstalling a recently installed Windows update if the timing indicates a possible connection.
4. Use recovery media to access diagnostic tools if the built-in recovery environment is unavailable.
5. Consult a qualified technician if the system continues to fail or important data is at risk.

System Restore may reverse system changes made after the selected restore point. Review the changes before confirming the operation. Personal files are generally not the target of System Restore, but a separate backup remains important.

How to Verify the Problem Is Resolved

• Windows completes startup without displaying the 0x00000066 stop code.
• The computer can restart and shut down normally.
• The system remains stable during ordinary use.
• Relevant SFC, DISM, or disk-check results no longer report unresolved problems.
• Hardware diagnostics do not report relevant failures.
• No recurring crash dump shows the same bug check.

If the blue screen returns, record the new stop code and preserve the latest dump file. A different stop code may indicate a separate problem or provide additional evidence about the underlying failure.

Frequently Asked Questions

What is error 0x00000066 in Windows?

It is the Windows bug check CACHE_INITIALIZATION_FAILED. It indicates a failure associated with cache initialization and is documented by Microsoft as a very infrequent stop code.

Does CACHE_INITIALIZATION_FAILED mean the processor cache is damaged?

No. The stop-code name alone does not establish physical damage to the processor's L1, L2, or L3 cache. Further diagnostic evidence is required before identifying a hardware component as defective.

Can a driver cause CACHE_INITIALIZATION_FAILED?

A driver change is a reasonable area to investigate when the crash begins after installing or updating low-level system software. However, Microsoft does not identify a specific driver as the universal cause of this bug check.

Can SFC fix the 0x00000066 blue screen?

SFC can repair certain corrupted protected Windows system files. It may help if system-file corruption is involved, but there is no guarantee that it will resolve this particular bug check.

What should I do if Windows keeps restarting?

Enter Windows Recovery Environment and try Startup Repair. If necessary, use Safe Mode, System Restore, or other available recovery options. Back up important data before performing operations that may affect the Windows installation.

Is CACHE_INITIALIZATION_FAILED a RAM problem?

Not necessarily. Memory instability can be investigated as part of hardware troubleshooting, but the 0x00000066 code by itself does not confirm a RAM fault.

Where can I find the Windows crash dump?

Small memory dumps are commonly stored in C:\Windows\Minidump. Larger memory dumps may be stored as C:\Windows\MEMORY.DMP, depending on the configured Windows dump settings.

Conclusion

CACHE_INITIALIZATION_FAILED (0x00000066) is an uncommon Windows startup-related bug check. Because Microsoft's public documentation provides limited diagnostic detail for this error, troubleshooting should focus on collecting crash evidence and investigating recent system changes rather than assuming a particular hardware or software fault.

Start with nonessential peripheral removal, Windows Recovery Environment, Startup Repair, and appropriate system-file checks. If the crash persists, examine the dump file and investigate hardware stability with suitable diagnostic tools.

Sources

• Microsoft Learn – Bug Check 0x66: CACHE_INITIALIZATION_FAILED
• Microsoft Learn – Bug Check Code Reference
• Microsoft Support – Troubleshoot blue screen errors
• Microsoft Support – Use the System File Checker tool to repair missing or corrupted system files
• Microsoft Learn – Repair a Windows Image
• Microsoft Learn – Analyzing a Kernel-Mode Dump File with WinDbg