[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.

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.

Step 6: Click on Advanced.

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

Step 8: 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.

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:

• KB972363
• KB973709
• KB972581
• KB974234

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.

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.

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.

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.

Step 5: Restart your PC.