[Solved] How to Fix Windows Update encountered an unknown error Code 646

Windows users reported that certain updates failed to install with the unknown error Code 646 happen to Windows 7 and Windows 8.1. The detail error message as below:

Some updates were not installed
Error(s) found:
Code 646 Windows Update encountered an unknown error.

[Solved] How to Fix Windows Update encountered an unknown error Code 646
[Solved] How to Fix Windows Update encountered an unknown error Code 646
Follow the solution steps below to fix Error Code 646.

Content Summary

Solution 1: Execute Windows Update Troubleshooter
Solution 2: Install via Windows Update Catalog
Solution 3: Force to Start Windows Update and Background Intelligent Transfer services
Solution 4: Clear the AppData Key via Registry Editor

Solution 1: Execute Windows Update Troubleshooter

Step 1: Pressing Windows key + R to open a Run box.

Step 2: Type control and press Enter to open the Classic Control Panel interface.

Step 3: Type troubleshoot in the search bar located at the top-right corner of the screen.

Step 4: From the list of search results, click on the Troubleshooting entry.

Step 5: Click on Fix problems with Windows Update under System and Security to open the Windows Update Troubleshooter.

Click on Fix problems with Windows Update under System and Security to open the Windows Update Troubleshooter.
Click on Fix problems with Windows Update under System and Security to open the Windows Update Troubleshooter.

Step 6: Click on Advanced.

Click on Advanced.
Click on Advanced.

Step 7: Check the box associated with Apply repairs automatically.

Check the box associated with Apply repairs automatically.
Check the box associated with Apply repairs automatically.

Step 8: Click on Apply this Fix and follow the on-screen prompts.

Click on Apply this Fix and follow the on-screen prompts.
Click on Apply this Fix and follow the on-screen prompts.

Step 9: Restart your PC.

Solution 2: Install via Windows Update Catalog

Step 1: Visit Microsoft Update Catalog.

Microsoft Update Catalog
Microsoft Update Catalog

Step 2: Search for the failed update KB using the search function located at the top-left corner of the screen. In most cases, the error code 646 is confirmed to occur with the following Windows Updates:

Search for the failed update KB using the search function located at the top-left corner of the screen.
Search for the failed update KB using the search function located at the top-left corner of the screen.

Step 3: Download the appropriate Windows Update according to the CPU architecture and the Windows OS version.
Step 4: Click on the .inf file and choose Install from the context menu.

Click on the .inf file and choose Install from the context menu.
Click on the .inf file and choose Install from the context menu.

Step 5: Restart your PC after Windows Update installs completed.

Solution 3: Force to Start Windows Update and Background Intelligent Transfer services

Step 1: Press Windows key + R to open a Run box.

Step 2: Type services.msc and press Enter to open the Services console.

Step 3: Located Windows Update Service, right-click on it and choose Properties from the context menu.

Step 4: Select the General tab, change the Startup type to Automatic, and click on the Start button.

Located Windows Update Service, right-click on it and choose Properties from the context menu. Select the General tab, change the Startup type to Automatic, and click on the Start button.
Located Windows Update Service, right-click on it and choose Properties from the context menu. Select the General tab, change the Startup type to Automatic, and click on the Start button.

Step 5: Click on the Apply button to save the changes.

Step 6: Located Background Intelligent Transfer Service, right-click on it and choose Properties from the context menu.

Step 7: Select the General tab, change the Startup type to Automatic, and click on the Start button.

Located Background Intelligent Transfer Service, right-click on it and choose Properties from the context menu. Select the General tab, change the Startup type to Automatic, and click on the Start button.
Located Background Intelligent Transfer Service, right-click on it and choose Properties from the context menu. Select the General tab, change the Startup type to Automatic, and click on the Start button.

Step 8: Click on the Apply button to save the changes.

Step 9: Restart your PC.

Solution 4: Clear the AppData Key via Registry Editor

Step 1: Press Windows key + R to open a Run box.

Step 2: Type regedit and press Enter to open up the Registry Editor. Click Yes to grant administrative privileges when prompted by the UAC (User Account Control).

Step 3: Navigate to the following location:
HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

Step 4: Double-click on the AppData registry key to ensure the value is %USERPROFILE%\AppData\Roaming. In case the value is different then modify it and click the OK to save the changes.

Double-click on the AppData registry key to ensure the value is %USERPROFILE%\AppData\Roaming. In case the value is different then modify it and click the OK to save the changes.
Double-click on the AppData registry key to ensure the value is %USERPROFILE%\AppData\Roaming. In case the value is different then modify it and click the OK to save the changes.

Step 5: Restart your PC.